feat: Spanner.V1 BatchWrite

[robertvoinescu-work]

User need access to the BatchWrite API through the Spanner SDK. This is important because BatchWrite allows users to submit a set of bulk mutations in batch for efficient processing. To address this, We introduce a BatchWriteAsync method in PooledSession exposing the API in Spanner.V1.

Details

• BatchWrite does not exist in a transaction managed by the client - all transaction logic is server side. So, the BatchWriteAsync method avoids transaction management.
• Integration tests have been added
• Unit testing has been avoided since the returned type from SpannerClient.BatchWrite, AsyncStream<>, is a sealed class (i.e. cannot be mocked) with no public constructor (i.e. cannot be explicitly created in our tests). We could introduce wrappers to allow for mocking, but that adds a lot of abstraction layers I'm not sure we want.

Non Goals

• Incorporate spanner into the Spanner ADO layer, this will be handled by another PR.

[amanda-tarafa]

I've left all the comments, but I think we should wait for MUX to implement this, let's chat. I'm marking as do not merge for the time being.