Populate spans in new payload

[fractalwrench]

Goal

Populates spans in the new payload. This isn't hooked up to the orchestrator yet but will allow replacing the old payload when that happens. I've taken the same logic from PayloadMessageCollator for gathering spans & transplanted it here.

Testing

Updated existing test coverage.

[codecov]

Codecov Report

Attention: Patch coverage is 89.47368% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.84%. Comparing base (8191745) to head (8d864fc).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
+ Coverage   79.80%   79.84%   +0.03%     
==========================================
  Files         409      409              
  Lines       10933    10939       +6     
  Branches     1614     1615       +1     
==========================================
+ Hits         8725     8734       +9     
+ Misses       1564     1561       -3     
  Partials      644      644              

Files	Coverage Δ
...pture/envelope/session/SessionPayloadSourceImpl.kt	91.66% <90.90%> (+8.33%)	⬆️
.../android/embracesdk/internal/payload/SpanMapper.kt	78.94% <87.50%> (ø)

... and 1 file with indirect coverage changes

[lucaslabari]

LGTM.
You can wait for other reviews if you need to validate payload-specific fields.

[priettt]

LGTM!