flexible message Content-Type handling

[Lance-Drane]

On the INTERSECT-SDK side, the eventual plan is for the message body to be only the raw data being sent, and having all metadata stored on protocol-level headers. This will allow for us to bypass a number of JSON encoding/decoding steps which must currently take place.

• switch from Server-Sent-Events (requires UTF-8 strings using the \n character as a delimiter) to Websockets ; this will be needed for certain data which is sent as a raw binary blob
• encode the entire message metadata (into i.e. JSON) and write it in before the final message delimiter
• (MAYBE:) have an additional "source" field as we currently have and evaluate that eagerly. This allows us to skip encoding/decoding JSON unless interacting directly with the message broker.

Ref: INTERSECT-SDK/python-sdk#8