gRPC spec and codec update - add support for arrow metadata

[zehiko]

What

Metadata associated with remote store recording will have some mandatory fields and additional arbitrary fields that we represent as arrow single row record batch.

When fetching metadata from the store about the recording, we expect to get back just the single row record batch that will have both the mandatory columns and any additional columns as defined during registration and subsequent updates.

We rely on codec for simple serialization and serialization of this data, which once again relies on arrow ipc.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!
• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

To run all checks from main, comment on the PR with @rerun-bot full-check.

[github-actions]

Deployed docs

Commit	Link
ff86342	https://landing-d4f4pyizi-rerun.vercel.app/docs

[jleibs]

This looks reasonable to me.