Optional content field in Chat Completions Request

[saileshd1402]

Currently, TGI is validating that "content" field always exists in chat completions request. But in case of an "assistant" message type in OpenAI messages struct, content can be empty.

According to OpenAI spec: "The contents of the assistant message. Required unless tool_calls or function_call is specified."

Example:

{
    "model": "llama-3-1-8b",
    "stream": false,
    "messages": [
        {
            "role": "assistant",
            "tool_calls": [
                {
                    "id": "0",
                    "function": {
                        "arguments": "{\"location\":\"Boston\",\"unit\":\"celsius\"}",
                        "name": "get_current_weather"
                    },
                    "type": "function"
                }
            ]
        }
    ]
}

Error returned by TGI:

Failed to deserialize the JSON body into the target type: messages[0]: missing field `content` at line 17 column 9

This PR fixes that by making content field optional and adding tool_calls to the Message struct (used in ChatRequest)