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 94.54%. Comparing base (126990b) to head (8b6e698).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2875      +/-   ##
==========================================
+ Coverage   94.48%   94.54%   +0.05%     
==========================================
  Files         486      488       +2     
  Lines       10405    10500      +95     
  Branches     1597     1613      +16     
==========================================
+ Hits         9831     9927      +96     
+ Misses        574      573       -1     

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