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/io-tests/configs/expected/no-defaults-with-db.config

@@ -13,6 +13,7 @@ db-config = "true"
 db-tx-end = "commit-allow-override"
 db-uri = "<REPLACED_WITH_DB_URI>"
 db-embed-default-join = "inner"
+db-use-legacy-gucs = false
 jwt-aud = "https://example.org"
 jwt-role-claim-key = ".\"a\".\"role\""
 jwt-secret = "OVERRIDEREALLYREALLYREALLYREALLYVERYSAFE"