feat: Add HeyGen as supported provider

[SecurityQQ]

Summary

Add HeyGen as a supported provider in the SDK, complementing the gateway integration (see gateway issue).

Tasks

• Add HeyGen provider definition
• Define HeyGen action types and Zod schemas
• Add HeyGen model definitions (avatars, voices, etc.)
• Update CLI to list and use HeyGen models
• Add HeyGen examples

References

• HeyGen API docs: https://docs.heygen.com/
• Gateway counterpart issue: vargHQ/gateway (linked)