Support setting endpoint for Google Cloud Storage (GCS) in Thanos

[antcs]

Is your proposal related to a problem?

We need to override the Google API endpoint used by Thanos for GCS storage to ensure usage of GCP regional endpoints.

Describe the solution you'd like

Proposal for a solution:

1. Add an option which can be used to configure the endpoint for the GCP storage client
2. Enhance the GCS client to set it using the option.WithEndpoint(...): https://github.com/grafana/thanos/blob/main/scripts/cfggen/main.go
3. It seems thanos-io/objstore is also used to support common actions with GCS. The functionality should also be added here: https://github.com/thanos-io/objstore/blob/main/providers/gcs/gcs.go

Describe alternatives you've considered

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Additional context

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[yeya24]

I think this is a valid proposal. Help wanted.