feat: get configuration parameters from the db

Allows configuring postgrest from the db by setting config parameters
on the connection role. For example:

ALTER ROLE postgrest_test_authenticator
SET pgrst.jwt-secret = "REALLYREALLYREALLYREALLYVERYSAFE"

The above wWill set the `jwt-secret` config option accordingly.

SUPERUSER privileges are required for ALTERing role settings,
so this might not work on some cloud-managed databases.

This feature is enabled by default, for disabling it you can add the
following to the config file:

db-load-guc-config = false

test/fixtures/schema.sql

@@ -1924,3 +1924,10 @@ $$ language sql;
 -- Only used for manually testing creating prepared statements
 create view prepared_statements as
 select * from pg_catalog.pg_prepared_statements;
+
+create or replace function change_max_rows_config(val int) returns void as $_$
+begin
+  execute format($$
+    alter role postgrest_test_authenticator set pgrst."db-max-rows" = %L;
+  $$, val);
+end $_$ volatile security definer language plpgsql ;