feat: expose TxBuilder coin selection

[reez]

Description

Exposes TxBuilder::coin_selection so people can choose the wallet coin selection algo when building transactions.

Notes to the reviewers

Looking for feedback on if we think this is something we want to do or not

Documentation

• bdk_wallet
  https://docs.rs/bdk_wallet/3.0.0/bdk_wallet/struct.TxBuilder.html#method.coin_selection
• bitcoin
• uniffi
• Other:

Changelog

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

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

[reez]

reviewable+rebased