[http-api] Consider collapsing message submission API endpoints

[mistermoe]

Currently, there are 3 endpoints to add a message to an exchange:

• POST /exchanges/:id/rfq
• POST /exchanges/:id/order
• POST /exchanges/:id/close

All of these endpoints expect a tbdex message as the request body. the tbdex message itself contains the message kind. Given this, I think we can probably collapse all of these into a single endpoint: POST /exchanges/:id/message.

Curious to hear what people think

[diehuxx]

In light of us changing the request structure of Submit RFQ aka Create Exchange, I think it's easier to keep these endpoints separate.