feat: add KMS extern example (Inception & Rotation) for KERIA PR #415

[yeomjaeseung]

Description

As requested by @kentbull, I have added a standalone example project in examples/ to demonstrate how the extern algorithm works with SignifyTS.

This example utilizes AWS KMS (Ed25519) as the external signing backend to validate:

1. Successful Inception: Creating an Extern AID with proper metadata.
2. Successful Rotation: Rotating the key and verifying the sequence update in the Key Event Log (KEL).
3. Metadata Persistence: Verifies that extern parameters (like extern_type) are correctly stored in KERIA and retrieved via identifiers().get(), resolving the previous 500 error.

[kentbull]

Great! I'll review this.