feat: Implement MultichainRouter and onProtocolRequest (SIP-26) (#2875)

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:
```json5
"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

---------

Co-authored-by: Maarten Zuidhoorn <[email protected]>

Author: FrederikBolding

packages/examples/packages/browserify-plugin/snap.manifest.json

@@ -7,7 +7,7 @@
     "url": "https://github.com/MetaMask/snaps.git"
   },
   "source": {
-    "shasum": "R4WjwqkDLNMUtU07n8AGq0WZKjsqjTjQXlASF++J4ws=",
+    "shasum": "Yzt/aRJTbRwAn3zbbK7W3MjgVVt2f6jLMGSc6pe2oyg=",
     "location": {
       "npm": {
         "filePath": "dist/bundle.js",

packages/examples/packages/browserify/snap.manifest.json

@@ -7,7 +7,7 @@
     "url": "https://github.com/MetaMask/snaps.git"
   },
   "source": {
-    "shasum": "06xWu+ehUlNMpbJQxTZi2mlnAJId3cLHEK6fWD2Z9rc=",
+    "shasum": "J+fHvBGBrcoSZ5R1dEiIO3PegqNGFElb1i4h9Zw4zxg=",
     "location": {
       "npm": {
         "filePath": "dist/bundle.js",

packages/snaps-controllers/coverage.json

@@ -1,6 +1,6 @@
 {
-  "branches": 93.06,
-  "functions": 96.61,
-  "lines": 98.08,
-  "statements": 97.8
+  "branches": 93.28,
+  "functions": 96.8,
+  "lines": 98.15,
+  "statements": 97.88
 }

packages/snaps-controllers/src/index.ts

@@ -5,3 +5,4 @@ export * from './utils';
 export * from './cronjob';
 export * from './interface';
 export * from './insights';
+export * from './multichain';