Consider allowing Snaps to listen / modify Web3 RPC calls

I imagine a situation where Metamask has not added new Web3 features and a Snap comes in and adds them themselves. If Snaps could listen to RPC requests made by the user, they could respond to them in a seamless way allowing to extend Metamask's RPC API.

Begin by reviewing the Snap RPC architecture and how user Web3 RPC requests currently flow through the project. Define the supported interception and modification behavior, including how a Snap responds, then identify the affected entry points and tests; done means the behavior is specified, implemented, and covered for representative RPC calls.

Written by the indexing model from the issue text.