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/unix-socket.config

@@ -1,3 +1,4 @@
 db-pool = 1
 server-unix-socket = "$(POSTGREST_TEST_SOCKET)"
 jwt-secret = "reallyreallyreallyreallyverysafe"
+db-load-guc-config = false