feat(block-scheduler): adds service and basic planner support for scheduler

[ashwanthgoli]

What this PR does / why we need it:

• Adds scheduler service with a basic run loop that reports lag metrics and plans jobs.
• Adds a job planner using time range based strategy

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

[owen-d]

Nice, looks like a good start. Left a couple small nits but happy to get this merged & iterate on top of it.

[owen-d]

tiny nit: this looks coupled to the GroupLag call -- let's instead implement a scheduler.grouplag() method which defers to the offset reader and afterwards updates metrics -- or just put the metrics in the OffsetReader itself. It's cleaner that way and more likely to be resilient to refactoring changes.