Propagate cache-operation user spans through CacheClient

[mar-cf]

Carry each cache operation's selected user parent in CacheClient metadata and forward it through the standalone cacheApiOutbound adapter alongside the internal span.

Verify that default and named match, put, and delete requests reach the backend in the caller's trace, parented on their own operation span rather than a new root or another cache operation.

[ask-bonk]

Since last review: 0 resolved, 0 still open, 0 new.
LGTM!

Not re-run: tests, api-compat, docs, compat-flags, design-simplicity, jsg-gc, kj-style, memory-safety (no author changes in their files since the last review)

---

_{Reviewed commit: 6c70585f · github run}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 7.93651% with 116 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.43%. Comparing base (e24a85c) to head (6c70585).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/workerd/api/tests/cache-span-parent-test-tail.js	0.00%	76 Missing ⚠️
src/workerd/api/tests/cache-span-parent-test.js	0.00%	40 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7655      +/-   ##
==========================================
- Coverage   38.46%   38.43%   -0.04%     
==========================================
  Files         864      868       +4     
  Lines      267608   267850     +242     
  Branches    25266    25269       +3     
==========================================
+ Hits       102936   102947      +11     
- Misses     150539   150770     +231     
  Partials    14133    14133              

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