Add demo for JEP 339 - Edwards-Curve Digital Signature Algorithm (EdDSA)

[Abhineshhh]

Description

Implements demo for JEP 339 - Edwards-Curve Digital Signature Algorithm (EdDSA) introduced in Java 15.

Fixes #286

Changes Made

• Added EdDSADemo.java with complete working examples:
  • Ed25519 key generation, signing, and verification
  • Ed448 key generation, signing, and verification
  • Demonstration of tampering detection
  • Comparison of Ed25519 vs Ed448 characteristics
• Added JEP 339 entry to JDK15Info.json (ordered first, before JEP 360)
• Registered demo in Java15.java initializer

Demo Output

The demo generates cryptographic keys, signs messages with both Ed25519 and Ed448, verifies signatures, and shows that tampered messages fail verification.

Checklist

• Follows contribution guide
• Proper Markdown-style documentation with JEP history and further reading
• Calls info(339) as first statement
• JSON entry added in correct numerical order
• Demo registered in Java15.java with // JEP 339 comment
• Working code examples with actual cryptographic operations
• No unrelated changes