Sort attribute tables by requirement level and name

[pyohannes]

Fixes #614

Changes

This changes the ordering of generated attribute tables to order by requirement level: required attributes first, then conditionally required ones, recommended ones, and opt-ins. The secondary sort criteria is attribute names.

This PR applies changes in open-telemetry/build-tools#260

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[lmolkova]

I believe this was addressed in the #820 and can be closed.