Add a GoogleAIGeminiDocumentEmbedder and GoogleAIGeminiTextEmbedder

[julian-risch]

We should enable users to use embedding models with the integration, for example text-embedding-004.
https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/text-embeddings-api
Came up on discord.

Related to #1530

[rhajou]

I can work on that as I already implemented it locally

[julian-risch]

Related publication https://deepmind.google/research/publications/157741/

[anakin87]

Google Generative AI SDK is deprecated and will reach EOL in September 2025.

Google GenAI SDK should be used instead, for which we already introduced Embedders in #1783.

For this reason, I am closing this issue and the related PR.