Design custom wallet import flow

[GBKS]

This is a follow-up to #62. We have the single-key flow nicely documented in the design docs. Let's also wrap up the custom wallet creation flow, for which we have some design explorations floating around (here and here).

It would be good to go through the usual steps:

• Discover: write some specs on what needs to be considered and do explorations
• Define: Whittle down explorations into a direction
• Design: Finalize the direction and figure out all the details
• Deliver: Document it all so it can go into implementation

Generally, this is for users who want a more custom wallet. Not entirely sure what all the options might be, but maybe they want to use a specific derivation path or address type, create a blank wallet without private keys, descriptor or legacy, etc. No need to hold back with options in this one.