Prepare 2.0.0 release (Part 1)

[joseharriaga]

• Implemented ChatMessageContent to encapsulate the representation of content parts in ChatMessage, ChatCompletion, and StreamingChatCompletionUpdate.
• Changed the representation of function arguments to BinaryData in ChatToolCall, StreamingChatToolCallUpdate, ChatFunctionCall, and StreamingChatFunctionCallUpdate.
• Renamed OpenAIClientOptions's ApplicationId to UserAgentApplicationId (commit_hash)
• Renamed StreamingChatToolCallUpdate's Id to ToolCallId
• Renamed StreamingChatCompletionUpdate's Id to CompletionId
• Replaced Auto and None in the deprecated ChatFunctionChoice with CreateAutoChoice() and CreateNoneChoice()
• Replaced the deprecated ChatFunctionChoice(ChatFunction) constructor with CreateNamedChoice(string functionName)
• Renamed FileClient to OpenAIFileClient and the corresponding GetFileClient() method in OpenAIClient to GetOpenAIFileClient().
• Renamed ModelClient to OpenAIModelClient and the corresponding GetModelClient() method in OpenAIClient to GetOpenAIModelClient().

[RogerBarreto]

ChatMessageContent triggers a conflict with our Semantic Kernel ChatMessageContent type, this change causes a very big impact on all of our customers.