feat(stovepipe): add Queue entity

[mnoah1]

Why?

Implementing process step.

This change creates a Queue entity to keep track of queue-level state. This matches the spec outlined in: https://github.com/uber/submitqueue/blob/main/doc/rfc/stovepipe/steps/process.md#queue-new-persisted-entity

What?

Add the per-queue Queue entity that backs the process stage's concurrency gate and backlog coalescing.

Test Plan

-Subsequent PRs will build out functionality that we can test

[behinddwalls]

⚠️ Automatic stack rebase failed

This PR could not be automatically rebased after its base PR was merged. The rebase hit conflicts that need manual resolution.

To fix manually:

git fetch origin
git checkout mnoah1/stovepipe-queue-entity
git rebase --onto origin/main c9acfbf57688aa0ed815fdfd64868d58fb64d1aa mnoah1/stovepipe-queue-entity
# resolve conflicts, then:
git push --force-with-lease

Then update this PR's base branch:

gh pr edit 309 --base main