Allow setting `retry-request` options

[stephenplusplus]

This library makes use of retry-request in streaming mode. However, retry-request allows options to be set which are not available through this library: https://github.com/googleapis/gax-nodejs/blob/82dfb83ca50535c48e0e31a43150e96c8e303077/src/streamingCalls/streaming.ts#L144

I'm not the best with this library -- @alexander-fenster, do you know how to make it possible so that from e.g. Bigtable, this would work:

const gaxClient = new v2[client](config);
const stream = gaxClient[method](reqOpts, gaxOpts, retryRequestOpts);

// or combined:
const gaxOpts = {
  retryRequestOptions: { ... },
};
const stream = gaxClient[method](reqOpts, gaxOpts);

[alexander-fenster]

I think we can add retryRequestOptions to gax options (your combined option above).