Consider BIP 87 for new wallets

[Sjors]

We currently use m/48h/0h/2h. The new BIP 87 suggests a simpler derivation path m/87h/0h.

This requires checking:

• if Specter supports it
• if other hardware wallet special case m/48h/0h/2h or if they don't care
• storing our path in Core Data, rather than hardcoding it in Signer.getSignerUs and SeedManager.ourPubKey (in order not to break existing wallets)

[w-i-n-s]

@Sjors On Core Data side we can store strings or data. So, no blockers here.