send message to assistant

[GuiBibeau]

there should be an endpoint to send a message to an assistant.

AC:

• the endpoint should be protected by a bearer token
• Ensure the user has the right permissions to send a message to the conversation
• ensure the last message is not from a human user, the AI should have time to respond before another message is sent
• Save the message
• return from the endpoint a simple success message. This will be updated later.