feat: add course versioning, credential display properties, and metadata update (#246, #245, #244, #243) - #376

Summary

This PR implements four feature enhancements to the ChainLearn smart contracts:

• closes . Add course prerequisites #246 - Course Prerequisites
  Already implemented in codebase. Verified that prerequisites field exists in Course struct with enrollment checks and queryability via set_prerequisites and get_prerequisites functions.
  closes . Add course versioning #245 - Course Versioning
• Added version: u32 field to Course struct in progress-tracker/src/types.rs
• Added completed_version: Option field to ProgressInfo and ProgressExport structs to track which version each learner completed
• Added update_course_version(course_id, new_version) admin function
• Tracks course version when learner becomes eligible for credential
  closes . Add credential display properties #244 - Credential Display Properties
• Added CredentialDisplay struct with optional image_url, description, and issuer_name fields
• Added CredentialVerification struct that combines CredentialInfo with optional CredentialDisplay
• Added Display(u64) variant to CredentialDataKey enum for storage
• Added set_credential_display and get_credential_display functions
• Added verify_credential_with_display function that returns both credential info and display properties
  closes . Add credential metadata update #243 - Credential Metadata Update
• Added update_credential_metadata(credential_id, new_metadata_uri) admin function
• Validates new URI using the same validation rules as minting
• Emits credential_metadata_updated event
• Ensures credential exists before allowing update
  All changes are backward compatible, include proper admin authorization checks, and follow existing patterns in the codebase.