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

[coveralls]

Pull Request Test Coverage Report for Build 18873961904

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Details

• 25 of 25 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 85.04%

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Totals
Change from base Build 18850737341:	0.05%
Covered Lines:	7026
Relevant Lines:	8262

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💛 - Coveralls

[thunderbiscuit]

If I understand correctly, this method relies on the wallet having access to the private keys in order to produce this map, but the new 3.0 wallet will not have access to those, so the method is not really useful for 3.0 users, nor as an example for how to perform signing when a wallet doesn't have private descriptors.

[notmandatory]

@GideonBature your approach isn't quite what we're looking for. Can you try updating this PR or creating a new one based on @ValuedMammal's recent comment on #331? see: #331 (comment)