Marketplace Listings API: buy, sell and verify listings

[buffrr]

This PR adds a walletbuy, walletsell and verifylisting RPC commands, and a standarized format for marketplace listings. This is simpler than having to deal directly with PSBTs and more human readable.

The listings look like this and can be verified using the verifylisting rpc call.

{
  "space": "@example",
  "price": 5000,
  "seller": "tbs1p6ly5949g3ga3x8sv2fgc7d8x2cs024sqw94yp7nh0nf788teqh3qrza2x6",
  "signature": "48e3d431e7fcbd7f04db5e96999665072484098fff8790dc596f64813f6d04545148218173497863d747920426ca688f9e8b82b9e5ac5ef08d9e22b2e9b8b3ac"
}

Using space-cli, selling a space is as simple as:

$ space-cli sell @example 5000

Buying a space

$ space-cli buy @example 5000 --seller <address> --signature <signature>

These listings are completely trustless!