google_vertex: mount integration under haystack_integrations.*

[masci]

Checklist

• Update the structure, the code, and examples on haystack-core-integrations
• Update the integration tile on https://github.com/deepset-ai/haystack-integrations Update google-vertex-ai.md haystack-integrations#151
• Update the docs at https://docs.haystack.deepset.ai/
• Update the notebooks on https://github.com/deepset-ai/haystack-cookbook (if needed) Update vertexai-gemini-examples.ipynb haystack-cookbook#29

[TuanaCelik]

Another todo to add is the blog, I'm updating it now, but jfyi for the next ones, some may have articles attached: https://haystack.deepset.ai/blog/gemini-models-with-google-vertex-for-haystack