Implement Abstraction Layer Code Competition model

What does this MR do and why?

As we are working on implementing Code Suggestion using Abstraction layer, we need to bring code_completion model in our code base. This MR is utilizing code-gecko model by adding the code_completion method to the VertexAI::Client.

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Before	After

How to set up and validate locally

1. Run the rails console rails c
2. Initialize the Vertex client client = Gitlab::Llm::VertexAi::Client.new(my_user)
3. Call the code_completion method with the valid content, e.g.

client.code_completion(content: { prefix: "def is_palindrome(string)", suffix: ""})

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #415581