feat(bdk_wallet): add get_keymap() helper method to get the KeyMap for Psbt.sign

[GideonBature]

Description

This PR adds a new helper method get_keymap() to the Wallet struct that returns a combined KeyMap containing all signing keys from both external and internal (change) signers.

Notes to the reviewers

Changelog notice

Added Wallet::get_keymap() helper method to retrieve a combined KeyMap from all wallet signers for convenient PSBT signing

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

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

Closes #331