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/SpecHelper.hs

@@ -91,6 +91,7 @@ _baseCfg = let secret = Just $ encodeUtf8 "reallyreallyreallyreallyverysafe" in
   , configDbConfig              = False
   , configDbUri                 = mempty
   , configDbEmbedDefaultJoin    = JTLeft
+  , configDbUseLegacyGucs       = True
   , configFilePath              = Nothing
   , configJWKS                  = parseSecret <$> secret
   , configJwtAudience           = Nothing
@@ -190,6 +191,9 @@ testCfgResponseHeaders testDbConn = (testCfg testDbConn) { configDbPreRequest =
 testMultipleSchemaCfg :: Text -> AppConfig
 testMultipleSchemaCfg testDbConn = (testCfg testDbConn) { configDbSchemas = fromList ["v1", "v2"] }
 
+testCfgLegacyGucs :: Text -> AppConfig
+testCfgLegacyGucs testDbConn = (testCfg testDbConn) { configDbUseLegacyGucs = False }
+
 resetDb :: Text -> IO ()
 resetDb dbConn = loadFixture dbConn "data"