feat: Full EPD Dynamo support

[indrajit96]

Overview:

Add support for full EPD for Image url in dynamo trtllm workers

Details:

• Encode Worker: Uses MultimodalEncoder to process images and generate disaggregated params with embedding handles
• Prefill Worker: Receives disaggregated params from Encode worker, handles prompts with tokens
• Decode Worker: Restores context including token IDs and max_tokens from Prefill worker
• Enhanced DisaggregatedParamsCodec to support multimodal embedding handles
• Updated engine initialization to conditionally use MultimodalEncoder vs LLM based on mode

Where should the reviewer start?

• request_handlers/handler_base.py - Core EPD pipeline flow logic
• request_handlers/handlers.py - EncodeHandler and PrefillHandler EPD support
• encode_helper.py - New process_encode_request for image URLs
• engine.py - MultimodalEncoder initialization