Add onSettingsPage export

[GuillaumeRx]

This PR adds a new onSettingsPage export that is used to display custom UI.

Changes:

• Adds a new export onSettingsPage: Similar to onHomePage this export allows you to show custom UI in MetaMask. Here it's used to display a custom section in MetaMask settings.
• Adds a new example snap (settings-page-example-snap) to demonstrate how to use this set of features and uses it in test-snap

Progresses: #2874

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.50%. Comparing base (b0036f5) to head (cbe344a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2911      +/-   ##
==========================================
+ Coverage   94.49%   94.50%   +0.01%     
==========================================
  Files         487      488       +1     
  Lines       10451    10473      +22     
  Branches     1604     1608       +4     
==========================================
+ Hits         9876     9898      +22     
  Misses        575      575              

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