Support Mistral API

[bilgeyucel]

Is your feature request related to a problem? Please describe.
We extend OpenAIChatGenerator for MistralChatGenerator. This works for chat completion but not for function calling. Mistral's function calling API is not the same as OpenAI's.
https://docs.mistral.ai/guides/function-calling/

Describe the solution you'd like
We need to support Mistral's own API in MistralChatGenerator

Describe alternatives you've considered
Not supporting function calling in Mistral

Additional context
With the current implementation, MistralChatGenerator states that a message is a function call but the content doesn't provide function name/argument, probably because we cannot map the response coming from Mistral to a ChatMessage object

{'replies': [ChatMessage(content='', role=<ChatRole.ASSISTANT: 'assistant'>, name=None, meta={'model': 'mistral-large-latest', 'index': 0, 'finish_reason': 'tool_calls', 'usage': {}})]}

[anakin87]

Related: we should probably also introduce a MistralGenerator which can play well with other components such as the AnswerBuilder

[AnushreeBannadabhavi]

I can work on this if no one is working on it!