fix: allow passing gax instance to client constructor (#629)

- [ ] Regenerate this pull request now.

PiperOrigin-RevId: 470911839

Source-Link: googleapis/googleapis@3527566

Source-Link: googleapis/googleapis-gen@f16a1d2
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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: googleapis/googleapis@d4a2367

Source-Link: googleapis/googleapis-gen@e97a1ac
Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiZTk3YTFhYzIwNGVhZDRmZTczNDFmOTFlNzJkYjdjNmFjNjAxNjM0MSJ9