Index xpub index for cosigners.

[nodech]

Even though we have path entry in database for cosigners it is not automatically used to figure out XPUB.index, but will be convenient to expose xpub.index to the users and simplify API usage.

Related bcoin-org/bcoin#689 - This tries to solve XPUB issue for bcoin xpub Only wallets.

[tynes]

This would greatly increase user experience, and we can use the xpub prefix bytes to make a good guess at the first few parts of the path. Right now to save some user experience, I pull the account index from each xpub returned from the api and then ledger.getPublicKey.xpubkey(network) until a match is found, then I know which account index to use when signing. This makes it so that the user doesn't need to enter their account index or path when signing, but there are cases when this will not work, like using a ypub/zpub, because bcoin doesn't support those yet, so its still possible to specify a path

[tynes]

If we had this bcoin-org/bcoin#616 merged into bcoin, it would make it much easier to infer the path based on the xpub

• we always know that it will be m/
• can use the xpub prefix to determine the purpose
  • this isn't 100% but people that are that custom probably write their own tools anyways
  • can account for xpub, ypub, zpub, tpub
• coin type could be derived from bcoin/lib/protocol/networks.[network].coinType
• account can be parsed from xpub
• branch + index are in the database