feat: add shard label splitter

[trevorwhitney]

What this PR does / why we need it:

This PR introduces a new splitter to use in volume queries, that splits the query by shard label in addition to time. The goal here is to create more parallelism between the queriers and the index gateways when executing volume queries.

TODO:

• performance evaluation before and after
• profiles before and after

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
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