feat(rpc): ability to retrieve xpub/zpub for each account

[theborakompanioni]

There seems to be "problems" with the xpubs contained in the /display response. The extended public keys represent the change "sub-account", e.g. m / purpose' / coin_type' / account' / change instead of m / purpose' / coin_type' / account'. This seems to be done intentionally, so do not see this as a bug report. (But I wonder, how useful they are in practice - keys derived from this are never used by JM, right?)

However, having the xpub/zpub for each mixdepth as distinct property in a response from the api would be awesome. 🙌
Jam wants to avoid to do the calculation on the front end and feed a library with the users seed just to get the extended public key.

Related #1267 ("Enable importing joinmarket wallet as watch-only to mobile wallets").

Any insights on this would be great. What are the downsides of this? Do benefits outweigh them?

[theborakompanioni]

Example wallet-tool.py output:

user@host:/src/scripts# ./wallet-tool.py demo.jmdat
JM wallet
mixdepth	0	fbonds-mpk-tpubDCMAFUjednydKPRZZJj177nEsXHzCYVktf4VZdJd2FpoMVhUrzXnv4EYktXrpss12SZVV7fFkTfPP7vVoxAiBzPUm99QMALXR3Ds9eL2qAc
external addresses	m/84'/1'/0'/0	tpubDDvCuyQC28XttRfpYu9oWeN9jTiGErAuBep6nn4s69xdPEbK9bLvNVipfpXfLYjgA1M5yk7kn5LEipzpJSmaDDQoE5S8VAu27NDjUPhWp4y

Example response from GET /display:

{
  "walletname": "demo.jmdat",
  "walletinfo": {
    "wallet_name": "JM wallet",
    "accounts": [
      {
        "account": "0",
        "account_balance": "0.00000000",
        "available_balance": "0.00000000",
        "branches": [
          {
            "branch": "external addresses\tm/84'/1'/0'/0\ttpubDDvCuyQC28XttRfpYu9oWeN9jTiGErAuBep6nn4s69xdPEbK9bLvNVipfpXfLYjgA1M5yk7kn5LEipzpJSmaDDQoE5S8VAu27NDjUPhWp4y",
            "entries": [...],
            "balance": "0.00000000",
            "available_balance": "0.00000000"
          },
         {
            "branch": "internal addresses\tm/84'/1'/0'/1\t",
            "entries": [],
            "balance": "0.00000000",
            "available_balance": "0.00000000"
          },
          {
            "branch": "internal addresses\tm/84'/1'/0'/2\ttpubDDvCuyQC28XtyuBCC7x1wMciYo39NA3nH47akxKSYoEHtQpadL8prDSrdaffg5f7nMPJrw97gEgeBD5F8DbW3rG4YYScwvres9qiLVGL1p9",
            "entries": [],
            "balance": "0.00000000",
            "available_balance": "0.00000000"
          },
          {
            "branch": "internal addresses\tm/84'/1'/0'/3\ttpubDDvCuyQC28XtztYY3tZCZSFF4YeBd4gs1jZndK9GVzJiMTjq3L89LsN3cyd1Dm2gGXojNn4oh7DMVQukcKTgWZ3swRZV7r2gMGyEzewQDaw",
            "entries": [],
            "balance": "0.00000000",
            "available_balance": "0.00000000"
          }
        ]
      },
      {
        "account": "1",
 [...]