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)

default.nix

@@ -36,6 +36,7 @@ let
       allOverlays.gitignore
       allOverlays.postgresql-default
       allOverlays.postgresql-legacy
+      allOverlays.postgresql-future
       (allOverlays.haskell-packages { inherit compiler; })
     ];
 
@@ -45,6 +46,7 @@ let
 
   postgresqlVersions =
     [
+      { name = "postgresql-14"; postgresql = pkgs.postgresql_14; }
       { name = "postgresql-13"; postgresql = pkgs.postgresql_13; }
       { name = "postgresql-12"; postgresql = pkgs.postgresql_12; }
       { name = "postgresql-11"; postgresql = pkgs.postgresql_11; }