FR: Allow override the default *.googleapis.com hostname when calling API

[john012343210]

Similar to firebase/firebase-android-sdk#905

Request is:
Hope the sdk could allow a proxy server domain to be set.

Reason:
since the default *.googleapis.com hostname is blocked in china, which my proxy server could be deployed in an environment that has access to *.googleapis.com

[zymotik]

Duplicate of #1641, you may find the comments there helpful.

[google-oss-bot]

Hey @sunset1234321. We need more information to resolve this issue but there hasn't been an update in 5 weekdays. I'm marking the issue as stale and if there are no new updates in the next 5 days I will close it automatically.

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