Add assistant.tool_choice parameter to force the Assistant to use a certain Tool function

[andreibondarev]

Currently the Assistant optionally picks a tool whenever it "thinks" it's appropriate to do so (tool_choice = "auto"). There are instances when you want the Assistant to use a specific tool 100% of the time (tool_choice = { "tool_name" }). This ticket was inspired by the following discussion: #733

Proposed changes:

• Introduce the @tool_choice instance variable that could be set during the initialization assistant = Langchain::Assistant.new(tool_choice: ...,) or after: assistant.tool_choice = "...".
• @tool_choice is used here, here and here.

If you take any of the built-in tools, you can see a list of that Tool's functions:

Langchain::Tool::NewsRetriever.function_schemas.to_openai_format.map { |h| h.dig(:function, :name) }
=> ["langchain_tool_news_retriever__get_everything", "langchain_tool_news_retriever__get_top_headlines", "langchain_tool_news_retriever__get_sources"]

This should be the value we're setting the tool_choice to:

assistant.tool_choice = "langchain_tool_news_retriever__get_sources"