WIP: Implement balance-fetching for derivable descriptors

[sgeisler]

This PR adds balance fetching based on miniscript descriptors and extended public keys. In its current form external and internal chains have to be fetched separately and added afterwards, which I think is reasonable.

This PR depends on rust-bitcoin/rust-miniscript#116, the following things have to happen for it to become merge ready:

• [ ] a new release of rust-bitcoin including rust-bitcoin/rust-bitcoin#443 has to be released
• [ ] rust-bitcoin/rust-miniscript#116 has to be finalized and depend on the newly release rust-bitcoin version
• [ ] a new version of rust-miniscript has to be released
• [ ] this PR has to be updated to depend on the new rust-bitcoin and rust-miniscript versions

[afilini]

Really nice!!

[notmandatory]

Hi, this is pretty old and I'm not sure this sort of functionality belongs in the electrum client. But if you still think it's a good idea and want to update it I'll see if I can get some smarter people than me to take a look.

[notmandatory]

Closing for now. If someone wants to pick it back up we can reopen.