[Easy] Add credential metadata storage

[Josie123-Dev]

Task Description

Implement credential metadata storage in contracts:

• Store credential metadata
• Update metadata
• Retrieve metadata
• Delete metadata
• Emit metadata events

Requirements

• Storage function
• Update function
• Retrieval function
• Deletion function
• Event emission

Acceptance Criteria

• Storage works
• Updates work
• Retrieval works
• Deletion works
• Events are emitted

[Williams-1604]

I'm relatively early in my journey with Soroban, but I've studied the codebase for Validfi and I believe I can handle this issue with care and attention to detail. I'd really appreciate the chance to prove myself here — I won't let the project down. Just like i did a perfect job last wave with no failing CI pipeline. Kindly consider my application please.