feat: StagedCommitter for easier Committer integration

[zachschuermann]

What changes are proposed in this pull request?

Adds a new StagedCommitter struct which effectively does the staged commit write for the engine and only requires the catalog RPC to be implemented.

How was this change tested?

todo

[codecov]

Codecov Report

❌ Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.61%. Comparing base (ac8510e) to head (d971c50).

Files with missing lines	Patch %	Lines
kernel/src/committer.rs	0.00%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1514      +/-   ##
==========================================
- Coverage   84.67%   84.61%   -0.06%     
==========================================
  Files         122      122              
  Lines       32741    32762      +21     
  Branches    32741    32762      +21     
==========================================
+ Hits        27722    27723       +1     
- Misses       3674     3695      +21     
+ Partials     1345     1344       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.