Adds core infrastructure for Git commit signing

[eamodio]

Implements the implementation plan for Phase 1 of Git commit signing support, establishing the foundational architecture for cryptographic signing within the extension.

• Core Infrastructure: Adds the ability to read and write signing configurations directly from Git global/local config, supporting GPG, SSH, and X.509 formats.
• Signature Parsing: Updates the commits provider to lazily fetch and parse signatures from git logs, verifying status (good, bad, expired, etc.) and ownership.
• Commit Creation: Enhances the patch provider to support signing when creating commits and unreachable commits, respecting the user's auto-sign configuration.
• User Interface: Introduces a new status bar controller to display the current signing mode and a setup command to help users configure signing keys.
• Integration: Hooks signing capabilities into the Composer feature, allowing generated commits to be signed.
• Telemetry & Error Handling: Implements specific error handling for common signing issues (missing keys, passphrase failures) and adds telemetry events to track adoption and stability.