[Medium] Implement Health Credential Registry contract

[Josie123-Dev]

Task Description

Create comprehensive Health Credential Registry contract:

• Register health credentials
• Store credential metadata
• Link credentials to wallets
• Implement credential lookup
• Add credential validation

Requirements

• Registration function
• Metadata storage
• Wallet linkage
• Lookup function
• Validation logic

Acceptance Criteria

• Registration works
• Metadata is stored
• Linkage is correct
• Lookup works
• Validation is implemented