Waterfalls PoC

[RCasatta]

Waterfalls is a new blockchain indexer which is optimized to be queried by descriptor or batch addresses.
Waterfalls started with liquid (it is serving https://liquidwebwallet.org/) and recently added support for bitcoin.

This is a proof of concept to integrate the waterfalls backend in bdk.

There are many open-points, shortcuts and bugs. But this is an end-to-end balance computation at least for 1 signet descriptor is the same as the electrum example

/git/bdk/examples/example_waterfalls$ cargo run -- init "tr(tpubDDh1wUM29wsoJnHomNYrEwhGainWHUSzErfNrsZKiCjQWWUjFLwhtAqWvGUKc4oESXqcGKdbPDv7fBDsPHPYitNuGNrJ9BKrW1GPxUyiUUb/0/*)" --change-descriptor 'tr(tpubDDh1wUM29wsoJnHomNYrEwhGainWHUSzErfNrsZKiCjQWWUjFLwhtAqWvGUKc4oESXqcGKdbPDv7fBDsPHPYitNuGNrJ9BKrW1GPxUyiUUb/1/*)' -n signet

$ cargo run -- scan
...
$ cargo run -- balance
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.15s
     Running `/home/casatta/git/bdk/target/debug/example_electrum balance`
confirmed:
    total             79690 sats
    spendable         79690 sats
    immature              0 sats
unconfirmed:
    total                 0 sats
    trusted               0 sats
    untrusted             0 sats

The purpose of this PR is to gather feedback and if there are no show-stopper work to make this mergeable in steps