feat: Implement MultichainRouter and onProtocolRequest (SIP-26)

[FrederikBolding]

This PR adds a MultichainRouter that can handle routing of non-EVM requests received via the multichain API. The multichain API should integrate this by calling MultichainRouter.handleRequest for any requests that are not understood by our existing JSON-RPC stack.

Additionally this PR implements onProtocolRequest, a new handler that Snaps can choose to register if they want to service protocol (non signing) requests for a given set of methods for one or more chains. The endowment is registered as follows:

"initialPermissions": {
  "endowment:protocol": {
    "scopes": {
      "<caip2_chainId>": {
        "methods": [
          // List of supported methods
        ],
        "notifications": [
          // List of supported notifications
        ]
      }
    }
  }
}

This implementation follows https://metamask.github.io/SIPs/SIPS/sip-26

Closes #2898

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.87%. Comparing base (c909011) to head (ef7e20c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2875      +/-   ##
==========================================
+ Coverage   94.80%   94.87%   +0.06%     
==========================================
  Files         503      506       +3     
  Lines       10996    11116     +120     
  Branches     1685     1702      +17     
==========================================
+ Hits        10425    10546     +121     
+ Misses        571      570       -1     

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[Mrtenz]

Maybe add a more descriptive error message?

[FrederikBolding]

Any proposals? 😄

[Mrtenz]

"Unable to get selected account" or something?

[FrederikBolding]

Done!