feat: get configuration parameters from the db

Allows configuring postgrest from the db by setting config parameters
on the connection role. For example:

ALTER ROLE postgrest_test_authenticator
SET pgrst.jwt-secret = "REALLYREALLYREALLYREALLYVERYSAFE"

The above wWill set the `jwt-secret` config option accordingly.

SUPERUSER privileges are required for ALTERing role settings,
so this might not work on some cloud-managed databases.

This feature is enabled by default, for disabling it you can add the
following to the config file:

db-load-guc-config = false

test/SpecHelper.hs

@@ -78,6 +78,7 @@ _baseCfg = let secret = Just $ encodeUtf8 "reallyreallyreallyreallyverysafe" in
   , configDbPreparedStatements  = True
   , configDbRootSpec            = Nothing
   , configDbSchemas             = fromList ["test"]
+  , configDbLoadGucConfig       = False
   , configDbUri                 = mempty
   , configJWKS                  = parseSecret <$> secret
   , configJwtAudience           = Nothing