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

[codecov]

Codecov Report

❌ Patch coverage is 88.37209% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.26%. Comparing base (bec219a) to head (7ac497f).

Files with missing lines	Patch %	Lines
src/wallet/mod.rs	88.37%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #355      +/-   ##
==========================================
+ Coverage   85.24%   85.26%   +0.01%     
==========================================
  Files          23       23              
  Lines        8230     8273      +43     
==========================================
+ Hits         7016     7054      +38     
- Misses       1214     1219       +5     

Flag	Coverage Δ
rust	85.26% <88.37%> (+0.01%)	⬆️

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