fix: allow passing gax instance to client constructor (#399)

- [ ] Regenerate this pull request now.

PiperOrigin-RevId: 470911839

Source-Link: https://github.com/googleapis/googleapis/commit/352756699ebc5b2144c252867c265ea44448712e

Source-Link: https://github.com/googleapis/googleapis-gen/commit/f16a1d224f00a630ea43d6a9a1a31f566f45cdea
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feat: accept google-gax instance as a parameter
Please see the documentation of the client constructor for details.

PiperOrigin-RevId: 470332808

Source-Link: https://github.com/googleapis/googleapis/commit/d4a23675457cd8f0b44080e0594ec72de1291b89

Source-Link: https://github.com/googleapis/googleapis-gen/commit/e97a1ac204ead4fe7341f91e72db7c6ac6016341
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