[WIP] Implement create_psbt for Wallet

[ValuedMammal]

Description

Use the new bdk_tx transaction building library to create PSBTs in BDK Wallet. Primary benefits include the use of bdk_coin_select as well as miniscript::plan module under the hood.

In this PR:

• Add method create_psbt for Wallet.
• Add module wallet::psbt_params
• Add example wallet/examples/psbt.rs

Notes to the reviewers

Still in draft but feel free to comment and make suggestions.

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines

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