Add support for Gemma model

[megatron6000]

Feature Description

This PR adds support for Gemma models hosted on GCP Model Garden

Added

• VertexAIGemmaModel class added

TODO:

• update documentation

[riccardo-alle]

@megatron6000 don't forget to update __all__ and get_available_models() to include Gemma. Actually, can we think about a test that would be trigger if we forget to list a new model? 🤔

[megatron6000]

@megatron6000 don't forget to update __all__ and get_available_models() to include Gemma. Actually, can we think about a test that would be trigger if we forget to list a new model? 🤔

I added a test that checks if models listed in get_available_models() are also included in __all__