feat(common): add custom predicate matching to RetryPolicy

[viacheslav-rostovtsev]

feat(common): add custom predicate matching to RetryPolicy

For Scotty we'll need to retry on non-HTTP/GRPC errors, e.g. on low-level socket timeout error. This seemed generic enough (and present in other languages e.g. python) to add to a RetryPolicy.

Adds a retry_predicate: keyword argument to Gapic::Common::RetryPolicy to support matching retry errors against a custom callable predicate.

Behavior

When a predicate is specified:

1. The predicate is executed first on the error.
2. If it returns nil, the decision on whether to retry is made on the basis of retry_codes.
3. Otherwise, the truthiness of the return value determines whether to retry.

Intentional Scope

Support for retry_predicate: was intentionally omitted from Gapic::CallOptions::RetryPolicy and Gapic::Operation::RetryPolicy.
We can add it there if needed. Scotty code will use the Gapic::Common::RetryPolicy code.