Add coin selection to tx_builder

[thunderbiscuit]

This one is particularly ugly. This is because the TxBuilder is generic in its coin selection algo, but we were only using 1 version of this generic and TxBuilder so far, so everything was easy.

I do think this could be cleaned up. I just wanted to run the tests and show where I'm at.

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Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[thunderbiscuit]

This way to handling coin selection is going away since the TxBuilder is being refactored for 3.0 and leveraging the bdk-tx crate instead. See bitcoindevkit/bdk_wallet#297 for development on this.

I think we're too close to 3.0 to make it worth our while to break everything, force users to update their code with the TxBuilder, just to then change it again in 3.0. This is not a requested/important feature enough to warrant the work IMO. We can instead focus on exposing the new transaction building methods once they come out.

Closing for now.