feat: add a config option for custom GCS endpoints

[na--]

What this PR does / why we need it:

This allows users to specify a custom endpoint for GCS buckets, which might be necessary for users to comply with data locality regulations by using custom regional endpoints.

Special notes for your reviewer:

This doesn't work if the thanos objstore client is used instead, that will be tackled in a separate PR. It should be safe to add in separate PRs because the GCS configs are different in either case.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[github-actions]

💻 Deploy preview deleted.

[ashwanthgoli]

lgtm