feat: public key generation from descriptors

[sdmg15]

Description

This PR as a new function to the wallet API public_key_at_index(keychain, index) which returns the public key found at the specified index. It closes #209

Notes to the reviewers

I explicitly matched all arms branches of the Descriptor<DefiniteDescriptorKey> so that if a new type is added in the future it signals there is a need to update this function too.

Changelog notice

• Add a helper function public_key_at_index(keychain, index) which returns the public key found at the specified index.

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