Add LLM ZeroShot Agent

What does this MR do and why?

Mentions https://gitlab.com/gitlab-org/gitlab/-/issues/412296

Hard coded Vertex chat endpoint to work with, not with OpenAI

user = User.first
client = Gitlab::Llm::VertexAi::Client.new(user)
context = Gitlab::Llm::Chain::GitlabContext.new(current_user: user, client: client, namespace: Group.first, resource: Issue.first)
agent = Gitlab::Llm::Chain::Agents::ZeroShot.new(input_prompt: 'What is the weather today?', tools: [], context: context)

puts agent.execute.content

I don't see how I can help. Please give better instructions!

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