Add a new constructor to the InboundGroupSession struct

[poljar]

As part of element-hq/element-meta#2720 we're going to add a new fields to the InboundGroupSession struct.

A new constructor will be helpful to initialize the new field when we receive a room key as an m.room_key event.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.82%. Comparing base (9de6d28) to head (27ed4ea).
Report is 54 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4688   +/-   ##
=======================================
  Coverage   85.82%   85.82%           
=======================================
  Files         292      292           
  Lines       33761    33761           
=======================================
+ Hits        28974    28975    +1     
+ Misses       4787     4786    -1     

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[andybalaam]

Great, thanks!