Suggest standard for combined signing challenge & connect request

Many sites ask the user to sign arbitrary data as an authentication step. Since all sites now have to request the user share their address, we could standardize a step that does both address exposure & arbitrary data signing in one step and make things smoother for dapps and users.

The issue names no files, tests, or implementation entry points. Start by locating the existing address-sharing and arbitrary-data signing request flows, then determine how a combined request would fit both dapps and users. Done would require an agreed standard and an implementation with tests covering the combined challenge.

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