feat(thanos): add support for named stores

[ashwanthgoli]

What this PR does / why we need it:

This pr adds named store support for thanos based object clients. -use-thanos-objstore flag has to be set to true to use these configs.

Below is an example of how to configure a named s3 store. store-1 can then be set as the object_store in your period_configs

storage_config:
  object_store:
    named_stores:
      s3:
        store-1:
          endpoint: s3://foo-bucket
          region: us-west1

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

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

[JoaoBraveCoding]

I'm a bit confused by this part. We set storeType using backend but then also use backend to search for a namedStore.