Add data models for the new call layouts

[robintown]

This is a start at implementing the call layouts from the new designs. I've added data types to model the contents of each possible layout, and begun implementing the business logic to produce these layouts in the call view model.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1bd76e0) 25.02% compared to head (87df543) 25.02%. Report is 2 commits behind head on livekit.

Additional details and impacted files

@@           Coverage Diff            @@
##           livekit    #2086   +/-   ##
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  Coverage    25.02%   25.02%           
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  Files           47       47           
  Lines         2382     2382           
  Branches       434      434           
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  Hits           596      596           
  Misses        1735     1735           
  Partials        51       51           

Flag	Coverage Δ
unittests	25.02% <ø> (ø)

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