[receiver/googlecloudmonitoring] Allow overriding default monitoring endpoint

[lhoursquentin]

Adds an endpoint property which overrides the default monitoring.googleapis.com:443.

This is needed when targeting non-standard universe domains. (e.g. S3NS: https://docs.cloud.google.com/sovereign-controls-by-partners/docs/data-boundaries/france-data-boundary-s3ns)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: lhoursquentin / name: Quentin L'Hours (433db3c, 731db33, 9771f1b)

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