refactor(storage): Checksum as an option

[dbolduc]

Part of the work for #2041

Fold Checksum into RequestOptions. This means we do not need to include Checksum as a parameter in the stub signatures.

The RequestOptions could/should hold an enum representing the checksum config that only gets turned into an engine when the engine is actually needed. We can do that later.

Also while we are here, make all the fields of RequestOptions crate-private. This gives us leeway to change the types if we make RequestOptions public.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.53%. Comparing base (98ea476) to head (f26c819).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3153      +/-   ##
==========================================
+ Coverage   96.50%   96.53%   +0.02%     
==========================================
  Files         108      108              
  Lines        4411     4410       -1     
==========================================
  Hits         4257     4257              
+ Misses        154      153       -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.