Update bdk dependency to 0.22

[thunderbiscuit]

The bindings do not build when attempting this upgrade because get_balance() now returns a Balance struct (this was merged in bitcoindevkit/bdk#640)

error[E0308]: mismatched types
   --> src/lib.rs:433:9
    |
432 |     fn get_balance(&self) -> Result<u64, Error> {
    |                              ------------------ expected `Result<u64, bdk::Error>` because of return type
433 |         self.get_wallet().get_balance()
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `u64`, found struct `Balance`
    |
    = note: expected enum `Result<u64, _>`
               found enum `Result<Balance, _>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `bdk-ffi` due to previous error

When we upgrade to 0.22.0 we could decide to add the Balance struct to the bindings, or simply return the total by calling get_total(), which returns a u64 (same as we have now).

[thunderbiscuit]

Because Balance is very simple (4 u64 fields) I decided to take a stab at bringing it in the FFI layer and test it in my test suite.

As is, the Balance struct works well, but the methods on it are not ported to the FFI layer. I'm not sure how to enable them without rewriting the struct in the FFI... @notmandatory any ideas? Would this be a case of using the typedef defined here?

I tried just for fun to add

[External="bdk"]
typedef extern Balance;

to the UDL file but unfortunatly it throws this error:

error[E0432]: unresolved import `bdk::FfiConverterTypeBalance`
    --> /Users/user/repos/bdk-ffi/target/release/build/bdk-ffi-28f1e274c858a20d/out/bdk.uniffi.rs:2621:5
     |
2621 | use bdk::FfiConverterTypeBalance;
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `FfiConverterTypeBalance` in the root

For more information about this error, try `rustc --explain E0432`.
error: could not compile `bdk-ffi` due to previous error

[notmandatory]

@thunderbiscuit I think we should make a bdk-ffi version of the Balance struct that also has fields for spendable and total. I can push a patch to this PR branch with the needed changes. This way we can leave Balance as a simple dictionary. Ok?

[thunderbiscuit]

ACK a6af35c. Ready for merging when the new BDK version comes out.

[notmandatory]

I rebased and updated the CHANGELOG. I also updated the PR title and changed this to a draft, once bdk 0.22 is published this PR can then be updated to replace the bdk 0.22.0-rc.1 version with the final 0.22 version.

[notmandatory]

Updated PR branch with final bdk release version 0.22. This should be ready to merge.

[notmandatory]

ACK 3c6075a