feat: Make GUC names for headers, cookies and jwt claims compatible with PostgreSQL v14

Getting the value for a header GUC on PostgreSQL v14 is done using `current_setting('request.headers')::json->>'name-of-header'` and in a similar way for `request.cookies` and `request.jwt.claims`

PostgreSQL versions below 14 can opt in to the new JSON GUCs by setting the `db-use-legacy-gucs` config option to false (true by default)

test/fixtures/schema.sql

@@ -82,7 +82,10 @@ CREATE FUNCTION set_authors_only_owner() RETURNS trigger
     LANGUAGE plpgsql
     AS $$
 begin
-  NEW.owner = current_setting('request.jwt.claim.id');
+  NEW.owner = case when current_setting('server_version_num')::int >= 140000
+              then current_setting('request.jwt.claims')::json->>'id'
+              else current_setting('request.jwt.claim.id')
+              end;
   RETURN NEW;
 end
 $$;
@@ -301,7 +304,10 @@ CREATE OR REPLACE FUNCTION switch_role() RETURNS void
 declare
   user_id text;
 Begin
-  user_id = current_setting('request.jwt.claim.id')::text;
+  user_id = case when current_setting('server_version_num')::int >= 140000
+            then (current_setting('request.jwt.claims')::json->>'id')::text
+            else current_setting('request.jwt.claim.id')::text
+            end;
   if user_id = '1'::text then
     execute 'set local role postgrest_test_author';
   elseif user_id = '2'::text then
@@ -329,18 +335,33 @@ CREATE FUNCTION reveal_big_jwt() RETURNS TABLE (
       iss text, sub text, exp bigint,
       nbf bigint, iat bigint, jti text, "http://postgrest.com/foo" boolean
     )
-    LANGUAGE sql SECURITY DEFINER
+    LANGUAGE plpgsql SECURITY DEFINER
     STABLE
     AS $$
-SELECT current_setting('request.jwt.claim.iss') as iss,
-       current_setting('request.jwt.claim.sub') as sub,
-       current_setting('request.jwt.claim.exp')::bigint as exp,
-       current_setting('request.jwt.claim.nbf')::bigint as nbf,
-       current_setting('request.jwt.claim.iat')::bigint as iat,
-       current_setting('request.jwt.claim.jti') as jti,
-       -- role is not included in the claims list
-       current_setting('request.jwt.claim.http://postgrest.com/foo')::boolean
-         as "http://postgrest.com/foo";
+    BEGIN
+    -- JWT claims are set in JSON format since v14
+    IF (current_setting('server_version_num')::INT >= 140000) THEN
+        RETURN QUERY
+        SELECT current_setting('request.jwt.claims')::json->>'iss' as iss,
+               current_setting('request.jwt.claims')::json->>'sub' as sub,
+               (current_setting('request.jwt.claims')::json->>'exp')::bigint as exp,
+               (current_setting('request.jwt.claims')::json->>'nbf')::bigint as nbf,
+               (current_setting('request.jwt.claims')::json->>'iat')::bigint as iat,
+               current_setting('request.jwt.claims')::json->>'jti' as jti,
+               (current_setting('request.jwt.claims')::json->>'http://postgrest.com/foo')::boolean
+                 as "http://postgrest.com/foo";
+    ELSE
+        RETURN QUERY
+        SELECT current_setting('request.jwt.claim.iss') as iss,
+               current_setting('request.jwt.claim.sub') as sub,
+               current_setting('request.jwt.claim.exp')::bigint as exp,
+               current_setting('request.jwt.claim.nbf')::bigint as nbf,
+               current_setting('request.jwt.claim.iat')::bigint as iat,
+               current_setting('request.jwt.claim.jti') as jti,
+               current_setting('request.jwt.claim.http://postgrest.com/foo')::boolean
+                 as "http://postgrest.com/foo";
+    END IF;
+END;
 $$;
 
 
@@ -1093,6 +1114,11 @@ create function test.get_guc_value(name text) returns text as $$
   select nullif(current_setting(name), '')::text;
 $$ language sql;
 
+-- Get the GUC values for Postgres v14.0 and up
+create function test.get_guc_value(prefix text, name text) returns text as $$
+select nullif(current_setting(prefix)::json->>name, '')::text;
+$$ language sql;
+
 create table w_or_wo_comma_names ( name text );
 
 create table items_with_different_col_types (
@@ -1784,8 +1810,13 @@ openapi json = $$
     }
   }
 $$;
+accept text;
 begin
-case current_setting('request.header.accept', true)
+accept = case when current_setting('server_version_num')::int >= 140000
+         then current_setting('request.headers', true)::json->>'accept'
+         else current_setting('request.header.accept', true)
+         end;
+case accept
   when 'application/openapi+json' then
     return openapi;
   when 'application/json' then
@@ -1958,9 +1989,15 @@ add constraint snd_shift foreign key           (snd_shift_activity_id, snd_shift
 -- for a pre-request function
 create or replace function custom_headers() returns void as $$
 declare
-  user_agent text := current_setting('request.header.user-agent', true);
+  user_agent text := case when current_setting('server_version_num')::int >= 140000
+                     then current_setting('request.headers', true)::json->>'user-agent'
+                     else current_setting('request.header.user-agent', true)
+                     end;
   req_path   text := current_setting('request.path', true);
-  req_accept text := current_setting('request.header.accept', true);
+  req_accept text := case when current_setting('server_version_num')::int >= 140000
+                     then current_setting('request.headers', true)::json->>'accept'
+                     else current_setting('request.header.accept', true)
+                     end;
   req_method text := current_setting('request.method', true);
 begin
   if user_agent similar to 'MSIE (6.0|7.0)' then