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/memory-tests.sh

@@ -12,6 +12,7 @@ export PGRST_DB_POOL="1"
 export PGRST_SERVER_HOST="127.0.0.1"
 export PGRST_SERVER_PORT="$pgrPort"
 export PGRST_JWT_SECRET="reallyreallyreallyreallyverysafe"
+export PGRST_DB_LOAD_GUC_CONFIG="false"
 
 trap "kill 0" int term exit