Enable Assistant Tool Function Methods to be updated

[Bodekaer]

@StephenHodgson do you know how to assign local methods to Assistant functions when using the RetrieveAssistantAsync() method, where the OpenAI server-side assistant already has defined functions?

I find the documentation unclear on this, and so far been unable to find a way to assign a method to an existing list of tools.

E.g. I would have expected something like:

_assistant = await _openai.AssistantsEndpoint.RetrieveAssistantAsync(assistantId);
foreach (var tool in _assistant.Tools){
   tool.Function.Method = myLocalMethod;
}

[StephenHodgson]

I'll add this along with ability to update tools to fix #298 and #303