Add taproot descriptor template, BIP-0086

[danielnordh]

The new descriptor templates in 0.26 are great, but we are missing taproot.
Please add functions similar to the ones already included for native segwit:

public static func `newBip84`(`secretKey`: DescriptorSecretKey, `keychain`: KeychainKind, `network`: Network)  -> Descriptor
public static func `newBip84Public`(`publicKey`: DescriptorPublicKey, `fingerprint`: String, `keychain`: KeychainKind, `network`: Network)  -> Descriptor

[notmandatory]

The template spec is BIP-0086: https://github.com/bitcoin/bips/blob/master/bip-0086.mediawiki

[vladimirfomene]

@notmandatory, can I take this up?