[feature request]:use electrum server for fee rate estimation

The fee rate appears to be a fixed value if not specified, such as `send` rpc.
I would like to request to use the electrum/electrs `blockchain.estimatefee` for a fee rate estimate.

Also, it would be nice if it could be considered the confidential transaction discounts.
https://github.com/ElementsProject/elements/pull/1317

## Steps to reproduce
My understand is that the fee used in the following commands is a fixed value.
```sh
lwk_cli wallet send --wallet w1 --recipient tlq1qqwe0a3dp3hce866snkll5vq244n47ph5zy2xr330uc8wkrvc0whwsvw4w67xksmfyxwqdyrykp0tsxzsm24mqm994pfy4f6lg:1000:144c654344aa716d6f3abcc1ca90e5641e4e2a7f633bc09fe3baf64585819a49
```

## Additional Context
As far as I have checked the code, the fee_rate is set to 100.0.
https://github.com/Blockstream/lwk/blob/d5899a83cf21cd7d4a1ee2b3985fcdc4df4248b4/lwk_wollet/src/tx_builder.rs#L79

This is a feature that I thought would be nice to have in the implementation to make lwk available in peerswap.
Now, we call directly on electrs' estimate fee.
https://github.com/ElementsProject/peerswap/pull/288