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