add sharding support for thanos compactor

[sockmister]

Q	A
Bug fix?	no
New feature?	yes
API breaks?	no
Deprecations?	no
Related tickets	no
License	Apache 2.0

What's in this PR?

This add sharding support for the Thanos compactor. The implementation is mostly based on the existing sharding support for the store component, with the difference that this allows users to specify label, which is important when sharding compactors.

Why?

Big thanos deployments with huge number of blocks will not be able to keep up with the compaction, and sharding it is the only solution.

Additional context

Checklist

• [x] Code meets the Developer Guide
• [x] User guide and development docs updated (if needed)
• [x] Related Helm chart(s) updated (if needed)