[V1] [Spec decode] Llama4 type eagle support in v1

[RonaldBXu]

This PR adds the capability for llama4-type eagle heads to be used for speculative decoding in vLLM v1. This is my first major PR in vLLM, so feedback is appreciated : )

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[sarckk]

cc @zixi-qi @morgendave

[RonaldBXu]

Ready for review now

[WoosukKwon]

@RonaldBXu Looks good to me overall. Could you please add a test? Also, is there any available EAGLE head we can test this on?