[vLLM] Native support for Mamba, SSM, and hybrid transformer models in vLLM V1

### Motivation. Mamba, SSM, and hybrid transformer models are an important path forward towards models that scale linearly with sequence length. vLLM currently supports many models of this class ([Jamba](https://github.com/vllm-project/vllm/pull/4115), [Mamba](https://github.com/vllm-project/vllm/pull/6484), [Codestral Mamba](https://github.com/vllm-project/vllm/pull/9292), [Falcon Mamba](https://github.com/vllm-project/vllm/pull/9325), [Bamba](https://github.com/vllm-project/vllm/pull/10909), [Zamba2](https://github.com/vllm-project/vllm/pull/13185), [MinimaxText01](https://github.com/vllm-project/vllm/pull/13454), [Plamo2](https://github.com/vllm-project/vllm/pull/14323)), and should continue to maintain excellent support for these models. **The Problem** SSM model generally are less-well supported than transformers in vLLM, and have several deficiencies. This RFC proposes several improvements (some already in progress) to SSM models, and additionally will serve as an issue tracker