feat: Implement SIP-31

[FrederikBolding]

Introduces the onClientRequest handler as specified in SIP-31. This handler can only be called by MetaMask and is available to all Snaps without permission. It can be used for general purpose communication that must not be exposed to dapps (as onRpcRequest).

This handler is enforced to only be callable by the metamask origin, as part of this PR this same validation is applied to all handlers in CLIENT_ONLY_HANDLERS.

Closes #3392

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.07%. Comparing base (fe8513a) to head (4bfe13a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3394   +/-   ##
=======================================
  Coverage   98.07%   98.07%           
=======================================
  Files         398      398           
  Lines       10966    10976   +10     
  Branches     1724     1728    +4     
=======================================
+ Hits        10755    10765   +10     
  Misses        211      211           

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