Useful descriptor functions

[Sjors]

• 1. Determine the descriptor type (e.g. sorted_multi()) and output type (legacy, p2sh segwit, native segwit, taproot)
• 2. Obtain the threshold for (sorted_multi()) and other such variables
• 3. Obtain the (extended) keys (done in descriptor: Implement wallet-policies BIP #369)

See #310

[jgriffiths]

@Sjors please see #369 - this allows key iteration and extracting all of their information such as type, derivation path, key origin info etc.

[jgriffiths]

Item 3 is now in master

[Sjors]

@jgriffiths I haven't been maintaining LibWallySwift much in recent months, so didn't test this, but I'll mark 3 as done in the description.