feat: Add multi-modal support vector store

[duwenxin99]

1. Add new image search APIs similarity_search_image() and asimilarity_search_image().
2. Add add_images() and aadd_images() endpoints to add images to vector store.
3. Add tests.

[averikitsch]

I manually tested this out and it's looking good. It might be helpful in your doc to describe the milestones of what is completed vs not completed ie GCS support.

Tagging in @kurtisvg as a python devex review on using both query/image_uris as inputs and how to handle differences in the embed_image interfaces while we sort out the interface with LangChain.

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[averikitsch]

Can you add the args to the doc string? We may also be able to call out the GCS support for the VertexAIEmbeddings model