feat: request.spec GUC for db-root-spec

The request.spec GUC contains the schema cache structure in json.

It's only available when the root endpoint(/) is requested and when
db-root-spec is not empty.

Also correct db-root-spec to accept a schema.

src/PostgREST/Query/SqlFragment.hs

@@ -32,7 +32,7 @@ module PostgREST.Query.SqlFragment
   , returningF
   , selectBody
   , sourceCTEName
-  , unknownLiteral
+  , unknownEncoder
   , intercalateSnippet
   ) where