Revamp Library: Improve Interfaces and Fix Functionality

[maxlambrecht]

This PR enhances the library's functionality and user experience.

Major enhancements include:

• Addressed issues with X509BundleSet and JwtBundleSet types to ensure they function as intended.
• Refactored SpiffeId and TrustDomain types to utilize __init__ constructors, streamlining the code in line with Pythonic standards.
• Transitioned getter methods to use the @property decorator, facilitating more straightforward attribute access.
• Updated method names for better clarity and usability in the library's public API.
• Expanded and refined test coverage for more robust validation.
• Integrated a check for the SPIFFE socket path in the Workload API client, preventing indefinite hangs and facilitating quicker error feedback.
• Updated workload.proto to reflect the latest specifications and regenerated protobuf code.

These changes aim to improve reliability, readability, and ease of use.