api: Add binding for `GET messages/{message_id}`

[chrisbobbe]

One wrinkle here is that GET messages/{message_id} doesn't give a whole message object until FL 120. Perhaps we can fill in that behavior by actually calling GET /messages for servers <120 (I think that accepts an id: NNNN narrow), then naturally clear out that fallback when we desupport servers <120. That fallback has the advantage of letting our binding support apply_markdown for all supported servers.

The binding can return the message object received from the server. If the message isn't found, we'll let the server's BAD_REQUEST error propagate.