feat(Core): add retryFunction in GrpcRequestWrapper::send

[vishwarajanand]

Core now allows adding retryFunction with GrpcRequestWrapper::send. This will be used in Spanner's BeginTransaction to retry for Not_FOUND errors with a custom pattern as followed in other languages.

Code Pointer in TS

A similar change is also being taken by Go, googleapis/google-cloud-go#7163

Change

1. Adding a retryFunction which takes in an Exception as argument and produces a bool to decide whether to retry or not.

2. Unit tests to validate cases such as whether retryLimit is still honored and follows the custom logic provided.

ref: b/263775077#comment8

[bshaffer]

looks good, one minor readability suggestion