Allow specifying credentials to vertexai embeddings model

[johanhaleby]

In 1.0.0-M2, the documentation for vertexai says to do this as part of the prerequisites:

gcloud config set project <PROJECT_ID> &&
gcloud auth application-default login <ACCOUNT>

However, it would be really useful to be able to specify credentials by other means, for example using a GCP service account. The vertex AI google cloud SDK supports this by passing an instance of com.google.auth.Credentials to com.google.api.gax.rpc.ClientContext used in PredictionServiceSettings. Builder. But in org.springframework.ai.vertexai.embedding.VertexAiEmbeddigConnectionDetails the predictionServiceSettings is hardcoded to:

PredictionServiceSettings.newBuilder().setEndpoint(endpoint).build()

making it very difficult to use a service account with spring ai.