Add TxBuilder control coin selection methods

[notmandatory]

Add methods to TxBuilder to allow users to control coin selection.

Note this issue requires Wallet.list_unspent() to be implemented first to get some OutPoints for testing.

See https://docs.rs/bdk/latest/bdk/wallet/tx_builder/struct.TxBuilder.html for docs on what the functions do.

interface TxBuilder {
  ... 
  TxBuilder add_unspendable(OutPoint unspendable);
  TxBuilder add_utxo(OutPoint outpoint);
  TxBuilder add_utxos(sequence<OutPoint>);
  TxBuilder do_not_spend_change();
  TxBuilder manually_selected_only();
  TxBuilder only_spend_change();
  TxBuilder unspendable(sequence<OutPoint>);
}

[zoedberg]

Really interested in this feature!

I was wondering if manually selecting UTXOs and setting manually_selected_only will be enough in order to set an absolute fee (that will be equal to the total input amount minus the total output amount) or if the fee_absolute method will be necessary in order to do that. Could you clarify this, please?

[notmandatory]

I believe that even if you use add_utxo and manually_selected_only you will also need to use fee_absolute to get an absolute fee since the coin selection will only use the specified utxos it needs to pay the outputs and some given fee or fee rate. You can see the rust docs here: https://docs.rs/bdk/latest/bdk/wallet/tx_builder/struct.TxBuilder.html#method.add_utxo

And if you want to test it out you can use the bdk-cli tool, the bdk-ffi behavior will be the same as bdk-cli. See the options with:

bdk-cli wallet -d <descriptor> help create_tx

[zoedberg]

I've implemented all the requested methods plus the fee_absolute one. Since this depends on the list unspent PR I think I should wait for that to be merged before opening a PR for this but feel free to assign me this issue in the meanwhile :)

[notmandatory]

You're also welcome to open a draft PR for some early feedback, but fine to wait until #158 is merged too.