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/io-tests/configs/no-defaults-env.yaml

@@ -11,6 +11,7 @@ PGRST_DB_PREPARED_STATEMENTS: false
 PGRST_DB_PRE_REQUEST: please_run_fast
 PGRST_DB_ROOT_SPEC: openapi_v3
 PGRST_DB_SCHEMAS: multi,   tenant,setup
+PGRST_DB_LOAD_GUC_CONFIG: false
 PGRST_DB_TX_END: rollback-allow-override
 PGRST_DB_URI: tmp_db
 PGRST_JWT_AUD: 'https://postgrest.org'