Add register command for BIP388 policies

[Sjors]

First step towards #785.

Usage:

hwi --device-type ledger register --name MooSig --desc "tr(musig(@0,@1)" --key [00000001/87'/0'/0']xpub... --key [00000002/87'/0'/0']xpub...

{"hmac": "00......"}

For MuSig2 you can compare the hmac with the result from MooSig.

Strongly overlaps with #647.

[Sjors]

Rebased after #795 landed.

[Sjors]

I'll give this a rebase after #836.

I need to sync this up with the latest #794 (e2a7288, c3815ba), and consider cherry-picking 562fe54.