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/expected/boolean-string.config

@@ -9,6 +9,7 @@ db-pre-request = ""
 db-prepared-statements = false
 db-root-spec = ""
 db-schemas = "required"
+db-load-guc-config = "false"
 db-tx-end = "commit"
 db-uri = "required"
 jwt-aud = ""