feat: accept google-gax instance as a parameter

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PiperOrigin-RevId: 474338479

Source-Link: googleapis/googleapis@d5d35e0

Source-Link: https://github.com/googleapis/googleapis-gen/commit/efcd3f93962a103f68f003e2a1eecde6fa216a27
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fix: allow passing gax instance to client constructor
PiperOrigin-RevId: 470911839

Source-Link: googleapis/googleapis@3527566

Source-Link: https://github.com/googleapis/googleapis-gen/commit/f16a1d224f00a630ea43d6a9a1a31f566f45cdea
Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiZjE2YTFkMjI0ZjAwYTYzMGVhNDNkNmE5YTFhMzFmNTY2ZjQ1Y2RlYSJ9

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: https://github.com/googleapis/googleapis-gen/commit/e97a1ac204ead4fe7341f91e72db7c6ac6016341
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fix: better support for fallback mode
PiperOrigin-RevId: 468790263

Source-Link: googleapis/googleapis@873ab45

Source-Link: https://github.com/googleapis/googleapis-gen/commit/cb6f37aeff2a3472e40a7bbace8c67d75e24bee5
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