Consider allowing Snaps to have more direct access to Metamask Infura infrastructure

Currently if Snaps want to support custom networks, they have to go out of band and use their own infrastructure to communicate with the network such as their own Infura keys. Having custom Infura keys in a public Snap is not the greatest idea. We could somehow allow Snaps to get access to Metamask's network infrastructure to be able to connect to IPFS, Polygon, Eth2, etc using Metamask provider.

CC @jeffprestes

No files, tests, or entry points are named. Start by reviewing how Snaps currently access custom networks and how the MetaMask provider exposes infrastructure, then clarify the security and scope requirements. Done requires an agreed implementation design for safe access to MetaMask services.

Written by the indexing model from the issue text.