Add support for the Mamba2 model.

[donpellegrino]

The Mamba2 model is used modularly in other models, such as IBM's Granite4, and would be useful to have available in rust-bert. For HuggingFace SafeTensors compatible weights see https://huggingface.co/AntonV/mamba2-130m-hf. See https://arxiv.org/pdf/[2405.21060](https://arxiv.org/pdf/2405.21060) for detilas on Mamba2. See https://www.ibm.com/new/announcements/ibm-granite-4-0-tiny-preview-sneak-peek for information on its use in another model.