Support for endpoint in rdsParams

[creatorrr]

Hi! I use the serverless framework and its plugin data-api-local-serverless for local development. The plugin generates starts a mock data-api service on a local endpoint.

At the moment, this package doesn't support setting a custom endpoint on the RDSDataService instance (see here). Please consider adding support for it? I'm happy to submit a PR.

[theBenForce]

Sounds like a good feature @creatorrr, if you create a PR I'll make sure it gets in.