getdescriptors: add --multipath

[Sjors]

Allow clients to obtain multipath descriptors, instead separate receive and change descriptors, by calling getdescriptors with --multipath.

So in addition to:

$ <cmd> --fingerprint f5acc2fd --chain test getdescriptors --account 0
{
  "receive": [
    "pkh([f5acc2fd/44h/1h/0h]tpubDCwY.../0/*)#...",
    "wpkh([f5acc2fd/84h/1h/0h]tpubDCtK.../0/*)#...",
    ...
  ],
  "internal": [
    "pkh([f5acc2fd/44h/1h/0h]tpubDCwY.../1/*)#...",
    "wpkh([f5acc2fd/84h/1h/0h]tpubDCtK.../1/*)#...",
    ...
  ]
}

Clients can use:

$ <cmd> --fingerprint f5acc2fd --chain test getdescriptors --account 0 --multipath
[
  "pkh([f5acc2fd/44h/1h/0h]tpubDCwY.../<0;1>/*)#...",
  "wpkh([f5acc2fd/84h/1h/0h]tpubDCtK.../<0;1>/*)#...",
  "sh(wpkh([f5acc2fd/49h/1h/0h]tpubDC87.../<0;1>/*))#...",
  "tr([f5acc2fd/86h/1h/0h]tpubDDKY.../<0;1>/*)#..."
]