Add message/mls MIME Type

[rohan-wire]

When MLS messages are transmitted over a protocol which supports Content-Type / MIME Type negotiation, signal that the message is an MLS message.

Because MLS messages are really a bag of formats today we should have a MIME type parameter which specifies the specific format included.

message/mls;format={application|handshake|welcome|keypackage|groupcontext}

[bifurcation]

This seems like a fine idea to me. Could go as a section in the protocol spec, or as a separate (small) spec.

[bifurcation]

Discussion on virtual interim:

• Section or separate doc?
• How do we handle versioning?
  • Internal to the protocol (field in MLSMessage?)
  • Field in the MIME type
  • Different MIME type per version

[rohan-wire]

• I would prefer a short section in the protocol document.
• I think Internal to the protocol is cleanest.
• Likewise the format parameter becomes unnecessary if we add Welcome, PublicGroupState, and KeyPackage to wire_format (see issue #574 )

[bifurcation]

@rohan-wire send a PR?

[seanturner]

Asking for friend: would we want to define (or use) a structured syntax suffix ala RFC 6839?

[bifurcation]

@seanturner If you mean for multiple formats, no, we have only one encoding. If someone wants to re-encode the protocol as XML or whatever, a MIME type will be the least of their troubles.

[bifurcation]

Fixed in #581