feat: Add Eagle3 speculative decoding support for Llama4

- Add Eagle3Llama4ForCausalLM model implementation
- Add SupportsEagle3 interface to Llama4ForConditionalGeneration
- Update eagle.py to support both Llama and Llama4 Eagle3 models
- Register Eagle3Llama4ForCausalLM in model registry

Signed-off-by: Rahul Tuli <[email protected]>

Author: rahul-tuli