API surface of the `TxBuilder` type

[thunderbiscuit]

After taking some time to look at what parts of the TxBuilder API were available/not available/useful for the bindings, many methods stood out. This issue will track them. Note that not all of those are top priority, but most of them we figured would be useful and not too costly to include either in the first version of the 1.0 or in a future feature release (1.1, 1.2, etc.)

• TxBuilder::add_data
• TxBuilder::current_height
• TxBuilder::nlocktime
• TxBuilder::coin_selection
• TxBuilder::ordering
• TxBuilder::sighash
• TxBuilder::allow_dust
• TxBuilder::policy_path
• TxBuilder::version

[thunderbiscuit]

Some notes related to the APIs I looked into:

1. TxBuilder::ordering

TxBuilder::ordering requires adding the TxOrdering type, which has the Custom variant:

Custom {
    input_sort: Arc<dyn Fn(&TxIn, &TxIn) -> Ordering>,
    output_sort: Arc<dyn Fn(&TxOut, &TxOut) -> Ordering>,
}

2. TxBuilder::sighash

TxBuilder::sighash requires adding the PsbtSighashType. I'm less familiar with this type, and the API does mention to be really careful when using the method. I'd like to be really sure I understand the PsbtSighashType and it's usage before opening this one up.

3. TxBuilder::coin_selection

TxBuilder::coin_selection requires adding the CoinSelectionAlgorithm.

[thunderbiscuit]

The TxBuilder is being replaced by bdk-tx in the 3.0 version of the library. I don't think we'll be putting much effort into exposing more complex things from the TxBuilder moving forward (since 3.0 is arriving in the fall).