feat(tracing): Propagate sample_rand to transaction's baggage

[szokeasaurusrex]

continue_trace now propagates incoming sample_rand values to the transaction's baggage. Also, in the case where sample_rand is missing from the incoming trace and needs to be backfilled, this change introduces a mechanism for the backfilled value from the scope's propagation context to be propagated to the transaction's baggage.

The transaction still does not use the sample_rand for making sampling decisions; this PR only enables propagation. A future PR will add support for reading the incoming/backfilled sample_rand and for using this value to make sampling decisions.

Depends on:

• feat(tracing): Backfill missing sample_rand on PropagationContext #4038

Ref #3998

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.36%. Comparing base (d486a8f) to head (b400fb1).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                        Coverage Diff                        @@
##           szokeasaurusrex/sample_rand-2    #4040      +/-   ##
=================================================================
+ Coverage                          80.30%   80.36%   +0.06%     
=================================================================
  Files                                140      140              
  Lines                              15545    15554       +9     
  Branches                            2635     2637       +2     
=================================================================
+ Hits                               12483    12500      +17     
+ Misses                              2225     2210      -15     
- Partials                             837      844       +7     

Files with missing lines	Coverage Δ
sentry_sdk/scope.py	85.57% <100.00%> (+0.03%)	⬆️
sentry_sdk/tracing.py	77.82% <100.00%> (ø)
sentry_sdk/tracing_utils.py	85.60% <100.00%> (+0.24%)	⬆️

... and 4 files with indirect coverage changes