Feat: Have SpanContext support the new W3C random flag.

[nikhilmantri0902]

Problem

• The Go SDK masks trace flags to the sampled bit and rejects version 00 flags > 0x02.
• This drops the W3C Trace Context Level 2 random Trace ID flag (0x02) and any forward-compatible bits.

Approach

• Treat trace flags as an 8-bit bitmask across extract, storage, and inject.
• Accept any flags for traceparent version 00 and preserve them unchanged.
• Interpret only known bits for behavior (sampling uses 0x01); do not change sampling logic.

Implementation details

• propagation/trace_context.go
  • Inject: emit sc.TraceFlags() without masking.
  • Extract: remove opts[0] > 2 rejection; store full flags byte in SpanContextConfig.TraceFlags.
• propagation/trace_context_test.go
  • Add extract cases for 0x02 and 0x03; ensure 0x09 is preserved.
  • Remove "unused bits set" from invalid cases.
  • Inject cases preserve 0xff, 0x02, and 0x03.
  • Future-version flag tests now expect full byte, not masked.

Closes #7635

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.7%. Comparing base (63b24b0) to head (0778d30).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7834     +/-   ##
=======================================
- Coverage   81.7%   81.7%   -0.1%     
=======================================
  Files        304     304             
  Lines      23252   23252             
=======================================
- Hits       19009   19007      -2     
- Misses      3860    3862      +2     
  Partials     383     383             

Files with missing lines	Coverage Δ
propagation/trace_context.go	96.9% <100.0%> (ø)

... and 1 file with indirect coverage changes

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[nikhilmantri0902]

Hi @dmathieu can you review this.

[dashpole]

Seems like we were doing this intentionally before. I'm hesitant to remove this behavior without understanding why it was added in the first place.