Add config flags to tell the tracer to create "runtime/trace" Task or Region

[nicohertz]

Instead of blindly creating a "runtime/trace".Task for every span, add config flags to control task and region creation

• for root local spans, keep creating a task
• for child local spans, do nothing unless explicitly asked to create a task or a region

Slack thread: https://cloud-native.slack.com/archives/C01NPAXACKT/p1763590922506999

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: nicohertz / name: Nico Hertz (109e8a3, 14a6c91, 2574968, 336be3f, 3a318e4, 3bbac2e, 40017b1, 4f7e5b9, 591db77, 595b3b8, 5e9cd73, 954ba51, 991bf23, a57a24d, b7858c0, cdcea7d, efd7924, f291643)

[codecov]

Codecov Report

❌ Patch coverage is 79.59184% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.1%. Comparing base (7323fc7) to head (efd7924).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
sdk/trace/runtime_trace.go	25.0%	6 Missing ⚠️
trace/config.go	75.0%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7628   +/-   ##
=====================================
  Coverage   86.1%   86.1%           
=====================================
  Files        298     299    +1     
  Lines      21694   21732   +38     
=====================================
+ Hits       18692   18727   +35     
- Misses      2626    2629    +3     
  Partials     376     376           

Files with missing lines	Coverage Δ
sdk/trace/span.go	87.2% <100.0%> (+1.7%)	⬆️
sdk/trace/tracer.go	100.0% <100.0%> (ø)
trace/config.go	94.2% <75.0%> (-2.6%)	⬇️
sdk/trace/runtime_trace.go	25.0% <25.0%> (ø)

... and 3 files with indirect coverage changes

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[dmathieu]

This PR is getting out of hand in terms of size. If you want us to be able to review this, you need to keep it small/atomic.
Also, this makes changes to the API with no related specification. So I think an RFC would be a better first step.