fix: Add helper for syncing a unity session to native reports

[kattrali]

Goal

Allow registering a session with the SessionTracker to be propagated to crash reports. This allows the session to be tracked separately via bugsnag-unity while preserving the stability score. The change is internally facing only.

Related to bugsnag/bugsnag-unity#138

Tests

Tested manually as a part of bugsnag/bugsnag-unity#138

Review

Outstanding Questions

• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

[fractalwrench]

This will need a changelog entry when the changes are complete.

[kattrali]

@fractalwrench Will it? Its entirely internally facing and documented in bugsnag/bugsnag-unity#138.

[fractalwrench]

@kattrali this PR alters source code so we should document the change, even though it isn't anticipated to affect Cocoa users. At least, that's my interpretation of the documented Platform Change Process.