Expose a getter or access to _client in the ServerlessClient class

[dangdennis]

I'd like to use Zapatos, pg ORM, with the benefits of ServerlessClient.

https://jawj.github.io/zapatos/#sql-tagged-template-strings

To use Zapatos, the .run method requires a pg.Client or pg.Pool. ServerlessClient itself cannot satisfy the pg.ClientBase interface.

I've already successfully made queries with Zapatos by ignoring the type checker and passing ._client. However, I'd like to do this without using ts-ignore everywhere.

[MatteoGioioso]

Hey, if you have time to make a PR that would be great, otherwise I think I could do something this coming weekend if I have time. Thanks