Allow dapps to signTypedData for non-selected networks

[danfinlay]

@jamesyoung requests the ability to use signTypedData 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 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.

[danfinlay]

As we build towards enabling connecting to multiple networks at once, it seems inevitable that we will have this feature in some form or another, so answering how we would feel comfortable enabling it safely could help answer questions about safe multi chain connections.

[danfinlay]

I personally would love if we could get the confirmations so obviously readable that a user would trivially tell the involved chain/asset, so enabling this could be safe.

[bschorchit]

Labeling this team-DevRel instead of team-transactions, but also flagging this #15705 as a blocker (PR is waiting for reviews to get merged) so all requests have a component to display the relevant network info (which is not the case currently).