Miscellaneous RPC changes
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- `getaddressinfo` now reports `solvable`, a boolean indicating whether all information necessary for signing is present in the wallet (ignoring private keys).
- `getaddressinfo`, `listunspent`, and `scantxoutset` have a new output field `desc`, an output descriptor that encapsulates all signing information and key paths for the address (only available when `solvable` is true for `getaddressinfo` and `listunspent`).