Stream retries

The NodeJS Firestore SDK currently has some logic that tries to detect whether a stream is healthy. If an error occurs before a stream has received a response message, the Firestore SDK retries its idempotent streams transparently.

I would like to explore the option of adding this retry behavior directly to GAX, which would allow us to simplify the Firestore SDK, increase the reliability for all other NodeJS SDKs and expose the same backoff behavior for all request types.

For reference, the Firestore implementation lives here: https://github.com/googleapis/nodejs-firestore/blob/master/dev/src/index.ts#L1170

A general retry mechanism for unary streams has recently been added to Python's GAX: https://github.com/googleapis/google-cloud-python/pull/10206