Add dedicated factory methods for Google Gemini vectorizer

[bevzzz]

This PR introduces dedicated factory method Multi2VecGoogleVectorizer::gemini that provides configuration options tailored to the Gemini API.

Additionally, we rename -Google methods to -GoogleVertex (similarly to how we have it in the generate/ module) and deprecate the older methods.

There's a slight breaking change in that Multi2VecGoogleVectorizer::vertex accepts VertexBuilder and not the base Builder. This is necessary to expose apiEndpoint setter for Vertex, but not for Gemini configuration. Users that use the lambda shorthand should not be affected at all; only explicit use of the builder type would need to be updated, e.g.:

Function<Multi2VecGoogleVectorizer.Builder, ObjectBuilder<Multi2VecGoogleVectorizer>> fn = opt -> opt.dimensions(768);
VectorConfig.multi2vecGoogle(fn); // Doesn't compile
VectorConfig.multi2vecGoogleVertex(fn); // Doesn't compile
VectorConfig.muli2vecGoogleVertex(builder -> builder.dimensions(768)); // Compiles as before

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