Allow API client and server to negotiate features

[abailly-iohk]

Why

Clients might have different requirements about the data they want from our API Server:

• Some might not want to be fed the history of previous messages when they connect
• Some might want to send/receive transactions in JSON or CBOR

What

• Provide a negotiation phase when a client connect to API Server
• Server sends Greeting message as before, possibly with a version
• Client sends a Greeting message with options set
• Server configures the connection according to the options.
• (optional): Server sends Available options in its initial Greeting message
• Then, we want to use this negotiation mechanism in the two example configurations we encountered so far:
  • Configurable transaction format
  • Configurable latching of history

How

• [ ] Create a negotiation mechanism
• [ ] #371
• [ ] #379

[ch1bo]

Another idea by @KtorZ is to use the sub-protocol negotiation in the WebSocket standard.