Operation polling now supports custom endpoints

[modular-magician]

Tests: I ran make testacc TEST=./google TESTARGS='-run=TestAccRedis' and got the following output: https://gist.github.com/smithson-google/92d8957671e9459424a1e5452eddf538. Tl;dr it passed on most tests. It only failed on tests which tried to use an API that hadn't been activated. I also ran a separate test with TF_LOG=DEBUG to make sure that HTTP requests were being sent as expected and they were. I didn't try to do my own testing with a custom endpoint, if necessary I can do that tomorrow.

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Fixed operation polling to support custom endpoints.

Derived from GoogleCloudPlatform/magic-modules#5610

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