feat: Allow passing btoa for mobile execution service

[FrederikBolding]

Allow passing in a btoa (base64 encoding) function to the mobile execution service. This lets us use native code for the encoding on mobile. We use base64 encoding to prevent XSS due to the weird nature of how React Native WebView postMessage works, but currently do not use a native implementation for base64. This is mostly problematic for larger messages, such as the executeSnap payload, that end up taking over half a second for some example Snaps. This problem would only grow for larger Snaps. We already use react-native-quick-base64 for other purposes in mobile, so dropping it into Snaps will be quite simple.

Old approach:
Base64 encoding took 523 ms

New approach
Base64 encoding took 1 ms

Progresses #2910

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.48%. Comparing base (c66e562) to head (ae074db).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2923   +/-   ##
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  Coverage   94.48%   94.48%           
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  Files         487      487           
  Lines       10428    10432    +4     
  Branches     1598     1599    +1     
=======================================
+ Hits         9853     9857    +4     
  Misses        575      575           

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