docs: Add new message structure to xCall design doc

[AntonAndell]

Description:

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Changelog Entry

version: <log entry>

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• I have performed a self-review of my own code
• I have documented my code in accordance with the documentation guidelines
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
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[AntonAndell]

I am not very happy with the digram that exists and also if you guys have any feedback in making the pseudo code sections clearer that would be good.

What diagrams of xCall would you like to see @nightowl121 @sabinchitrakar @redlarva

[codecov]

Codecov Report

Merging #193 (055416f) into main (052bb7d) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #193   +/-   ##
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  Coverage     68.83%   68.83%           
  Complexity       55       55           
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  Files            32       32           
  Lines          1925     1925           
  Branches         34       34           
=========================================
  Hits           1325     1325           
  Misses          584      584           
  Partials         16       16           

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solidity	65.02% <ø> (ø)

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[ibrizsabin]

looks good