Add `scantxoutset` method

[tvpeter]

And also scantxoutset

Originally posted by @luisschwab in #4

[luisschwab]

Blocked by rust-bitcoin/corepc#164

[oleonardolima]

It looks like the draft PR in corepc became stale. @luisschwab as you already implemented this in the old bitcoincore-rpc crate, I think it'd be best if you take over on that one if the PR author does not want to work on that anymore. WDYT ?

[luisschwab]

Sure, but may be a while before I get to it.