move one mimirpb custom type to separate package

[francoposa]

What this PR does

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].
• about-versioning.md updated with experimental features.

[pracucci]

I'm missing the context of this change. However, instead of calling the package mimirpb_custom have you considered naming the package based on the actual content, e.g. labelspb?

[francoposa]

I'm missing the context of this change. However, instead of calling the package mimirpb_custom have you considered naming the package based on the actual content, e.g. labelspb?

@pracucci it is not yet fleshed out - it appears making this a draft did not stop it from tagging a lot of people for review.

This PR is just part of an exploration of what it would take to move off of gogoproto - specifically here I am looking at removing the need for the custom type extension and still getting everything to compile & import correctly.

As far as package construction, we can look at specifics when this gets closer to an actual in project, but the mimirpb package is the only which exports custom proto types - specifically this LabelAdapter, PreallocatingMetric, PreallocTimeseries, and UnsafeByteSlice.
I just moved LabelAdapter here to prove out the process, but I figured it would be least disruptive but to keep those together especially as they all reference / build on each other.