Add new fields to SessionMessage class

[fractalwrench]

Goal

Adds new fields to the SessionMessage class. This will allow us to send the new style payload whilst maintaining backwards compatibility with the existing payload.

Testing

Added test coverage.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.16%. Comparing base (2f6c705) to head (cce78c5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   80.11%   80.16%   +0.04%     
==========================================
  Files         412      412              
  Lines       10989    10995       +6     
  Branches     1617     1618       +1     
==========================================
+ Hits         8804     8814      +10     
+ Misses       1538     1535       -3     
+ Partials      647      646       -1     

Files	Coverage Δ
...brace/android/embracesdk/payload/SessionMessage.kt	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[lucaslabari]

LGTM