mimirpb: Prepare for RW2.0 spec change

[alexweav]

What this PR does

This is a preliminary, internal PR that prepares the mimirpb package to uptake prometheus/prometheus#17411, which makes a breaking change to Remote Write 2.0 fields.

This PR does not add support for these fields directly, but it makes Mimir internally aware of them and proposes a path to handle the compatibility breakage. We don't listen to the new fields yet; as of this PR, clients sending start_timestamp will still continue to have the value ignored.

Official support for new fields will be added when Mimir's prometheus dependency is updated to pull in prometheus/prometheus#17411.

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

[alexweav]

I'm still working on test failures. There's a mixture of reasons - a handful are temporary, and are related to the new field being introduced ahead of the Prometheus one, and a couple others are only reproducing on the CI and not locally.

The worst effect is that we rely heavily on our message types being directly castable to promql.FPoint, but the underlying Prometheus PR removes this alignment - the RW2 write path sample has become a superset of the read path sample.

Likely we'll need to introduce a SampleRW2 instead, and not touch Sample.