Update client to support Electrum protocol v1.6 methods

### To Do

Comment below if this list is incomplete.

- [ ] `server.version` should tolerate and ignore extraneous arguments.
- [ ] Add optional mode argument for `blockchain.estimatefee`.
- [ ] `blockchain.block.headers` now returns headers as a list. Custom parsing logic can be implemented to support both the new and old responses.
- [ ] Add `blockchain.transaction.broadcast_package`.
- [ ] Add `mempool.get_info()`.

### Other notes

I think we should make this layer unopinionated for simplicity. The caller will be responsible for making the right calls. I.e. send `server.version` as the first message.

### Context

https://github.com/bitcoindevkit/rust-electrum-client/issues/189