Spring AI Add support for Custom HTTP Headers of Google GenAI

Well done on all the work for the google-genai library.

It would be great if we could specify custom HTTP headers for the ChatModel properties.

https://github.com/googleapis/google-cloud-java/blob/230fed0de5cf21f75da451643df383a469ee5a8b/java-vertexai/google-cloud-vertexai/src/main/java/com/google/cloud/vertexai/VertexAI.java#L263

Comment From: RyanHowell30

@markpollack in addition, I tried pulling in the library and setting my BOM version to 1.0.2 and nothing comes up in my gradle dependencies.