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)

nix/overlays/postgresql-future.nix

@@ -0,0 +1,17 @@
+self: super:
+# Overlay that adds future versions of PostgreSQL that are supported by
+# PostgREST.
+{
+  postgresql_14 =
+    let
+      rev = "76b1e16c6659ccef7187ca69b287525fea133244";
+      tarballHash = "1vsahpcx80k2bgslspb0sa6j4bmhdx77sw6la455drqcrqhdqj6a";
+
+      pinnedPkgs =
+        builtins.fetchTarball {
+          url = "https://github.com/nixos/nixpkgs/archive/${rev}.tar.gz";
+          sha256 = tarballHash;
+        };
+    in
+    (import pinnedPkgs { }).pkgs.postgresql_14;
+}