feat(wallet): add Wallet::sign_with_signers

[noahjoeris]

Description

Adds Wallet::sign_with_signers, allowing callers to pass signer containers. Wallet::sign now delegates to this new method, so the existing API remains unchanged.

This is a small step toward removing signer ownership from Wallet and improving separation of concerns. In the future we can deprecate Wallet::sign and remove the wallet signer fields.

Changelog notice

- Added `Wallet::sign_with_signers`.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.58%. Comparing base (6dca318) to head (24564ed).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
+ Coverage   80.55%   80.58%   +0.02%     
==========================================
  Files          24       24              
  Lines        5472     5479       +7     
  Branches      243      243              
==========================================
+ Hits         4408     4415       +7     
  Misses        987      987              
  Partials       77       77              

Flag	Coverage Δ
rust	80.58% <100.00%> (+0.02%)	⬆️

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