feat: add evictor

[grobinson-grafana]

What this PR does / why we need it:

This commit adds an evictor abstraction that will run the eviction of stale streams. The reason for abstracing it is two reasons:

1. It is easier to test
2. It will allow us to parallelize eviction over shards of metadata (for example, one per partition) to increase parallelism and reduce lock contention.

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