Support google.api_core.retry in googleapiclient.discovery.build?

It looks like `googleapiclient.discovery.build()` is super limited wrt to retry support is that correct?

In general is `googleapiclient.discovery.build` for legacy clients or APIs that Google doesn't support as fully? Some APIs use it and others have a specific library I can import and use `google.api_core.retry` with.

`Retry` has exponential backoff built into it, a bunch of sane defaults, and other features.

I generally just set a timeout value and let it use the default backoff settings which is so simple and effective for 99% of use cases I find.

Could the service client just support a Retry like this?

```
from googleapiclient.discovery import (build)
from google.api_core.retry import Retry

my_retry = Retry(timeout=30)
service = build("admin", "directory_v1", retry=my_retry)
```