feat: Add visit_string_map to the ffi

[nicklan]

What changes are proposed in this pull request?

Today we don't have a way to just visit all of a CStringMap. You rather have to probe one key at a time, and also somehow know all the keys ahead of time. This just allows visiting all the entries in one go.

How was this change tested?

New unit test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.42%. Comparing base (af6a5f5) to head (3bdca0e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1342      +/-   ##
==========================================
+ Coverage   84.39%   84.42%   +0.02%     
==========================================
  Files         112      112              
  Lines       27773    27819      +46     
  Branches    27773    27819      +46     
==========================================
+ Hits        23440    23486      +46     
+ Misses       3201     3199       -2     
- Partials     1132     1134       +2     

☔ 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.

[zachschuermann]

LGTM! one nit

[zachschuermann]

just dawned on me: also we could perhaps do something "nicer" by letting engines do fallible visiting? and then we stop early if the visitor e.g. returns false? but that's probably over-designing and we can add later if we need

.. disregard