Allow dapps to signTypedData for non-selected networks

@jamesyoung requests the ability to use [signTypedData](https://docs.metamask.io/guide/signing-data.html) for messages on non-selected networks.

This could be an easier lift than adding true multi-network support, since no data source is needed.

There is a danger that a user who believes they are on a low-stakes/test network is then presented with a higher-stakes confirmation.

One way to mitigate that danger would be to make this dapp ability a permission using [EIP 2255](https://github.com/ethereum/EIPs/issues/2255) permissions, like `Permission to sign messages for the X network`.

We would then also indicate the intended network on the signature screen.

Needs UI design and API design.

Start by reviewing the signTypedData documentation linked in the issue and EIP 2255 permissions. Define the UI and API requirements for signing on non-selected networks, including how the intended network is shown and how the permission or confirmation flow addresses the stated risk.

Written by the indexing model from the issue text.