Add permission for Snaps in order to receive JSON-RPC requests

[Mrtenz]

Initially, Snaps only had an RPC handler in order to provide any functionality. Since then, we've added other possibilities for Snaps to work, like transaction insights through the onTransaction entry point, and keyrings through the keyring entry point. Both of these entry points require a permission to work. We should do the same thing for the onRpcRequest entry point, requiring a permission in order for a Snap to be able to receive requests.

This also makes it possible for a Snap to specify whether it should be able to receive requests from just dapps, just other Snaps, or both. For example:

{
  "initialPermissions": {
    "endowment:rpc": {
      "dapps": true,
      "snaps": true
    }
  }
}

Both values can default to false. At least one must be true for a valid permission. For example:

• dapps: true, snaps: true - Requests from both other Snaps and dapps.
• dapps: true, snaps: false OR dapps: true - Only requests from dapps.
• dapps: false, snaps: true OR snaps: true - Only requests from other Snaps.
• dapps: false, snaps: false - Invalid. At least one of them must be true.