feat: add downscalePermittedFunc to check downscale is permitted

[grobinson-grafana]

What this PR does / why we need it:

This pull request adds a new abstraction downscalePermittedFunc which returns true if a downscale is permitted. It adds a function newChainedDownscalePermittedFunc that allows multiple funcs to be chained, and a newOffsetCommittedDownscaleFunc that checks if all records have been committed up to the end offset.

This prevents consumers from shutting down before processing all records in a partition as this would lead to data loss.

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

[grobinson-grafana]

Tested with a new topic (partition has no records):

level=info ts=2025-12-08T22:45:55.966684Z caller=downscale.go:38 component=dataobj-consumer msg="no records produced for partition"

Tested with a topic that has uncommitted records:

level=info ts=2025-12-08T22:46:26.342909Z caller=downscale.go:59 component=dataobj-consumer msg="there are uncommitted offsets" last_committed_offset=-1 end_offset=2 delta=2

Tested with a topic that has all records committed:

level=info ts=2025-12-08T22:49:20.018399Z caller=downscale.go:50 component=dataobj-consumer msg="all offsets have been committed" last_committed_offset=3 end_offset=4

End offset is the offset of the next record (the last produced record + 1).