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/Main.hs

@@ -36,6 +36,7 @@ import qualified Feature.HtmlRawOutputSpec
 import qualified Feature.IgnorePrivOpenApiSpec
 import qualified Feature.InsertSpec
 import qualified Feature.JsonOperatorSpec
+import qualified Feature.LegacyGucsSpec
 import qualified Feature.MultipleSchemaSpec
 import qualified Feature.NoJwtSpec
 import qualified Feature.NonexistentSchemaSpec
@@ -88,6 +89,7 @@ main = do
           (configDbSchemas config)
           (configDbExtraSearchPath config)
       appState <- AppState.initWithPool pool config
+      AppState.putPgVersion appState actualPgVersion
       AppState.putDbStructure appState customDbStructure
       when (isJust $ configDbRootSpec config) $
         AppState.putJsonDbS appState $ toS $ JSON.encode baseDbStructure
@@ -108,6 +110,7 @@ main = do
       responseHeadersApp   = app testCfgResponseHeaders
       disallowRollbackApp  = app testCfgDisallowRollback
       forceRollbackApp     = app testCfgForceRollback
+      testCfgLegacyGucsApp = app testCfgLegacyGucs
 
       extraSearchPathApp   = appDbs testCfgExtraSearchPath
       unicodeApp           = appDbs testUnicodeCfg
@@ -210,6 +213,10 @@ main = do
     parallel $ before multipleSchemaApp $
       describe "Feature.MultipleSchemaSpec" $ Feature.MultipleSchemaSpec.spec actualPgVersion
 
+    -- this test runs with db-uses-legacy-gucs = false
+    parallel $ before testCfgLegacyGucsApp $
+      describe "Feature.LegacyGucsSpec" Feature.LegacyGucsSpec.spec
+
     -- this test runs with db-embed-default-join = inner
     before embedInnerJoinApp $
       describe "Feature.EmbedInnerJoinSpecNotDefaultConfig" Feature.EmbedInnerJoinSpec.notDefaultConfig