Add streaming to Anthropic client

What does this MR do and why?

Add streaming to Anthropic client

This adds the option to stream data from Anthropic by calling the stream method. We added a new method instead of adding an option to complete because streaming does not work with the concept of exponential backoff.

It also adds the anthropic-version header with '2023-06-01' as Anthropic changed the way streaming works and the previous version will be deprecated. The old version accumulated the response in the stream, while the new version only sends the next token.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Make sure you have an Anthropic Token set in ApplicationSettings
2. Execute the following in the rails console:

   Gitlab::Llm::Anthropic::Client.new(User.first).stream(prompt: "Write a hello world in ruby:") do |f|
      puts f["completion"]
   end

MR acceptance checklist

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