Version negotiation about mls-protocol

[bifurcation]

It seems like the right answer here is roughly the same as for ciphersuites -- advertise support in UserInitKey, select in Welcome.

[bifurcation]

For -04, this will be a preliminary effort. @beurdouche points out that there are more subtle considerations here, e.g., what remains constant across versions. But we'll go ahead and put something basic in -04, as a basis for discussion.

[bifurcation]

Moving to -05 to see if we need something more.

[bifurcation]

Per discussion at interim, this is adequately addressed by:

• Stating a version in ClientInitKey (in current draft)
• Providing a list of supported versions in CIK to prevent downgrade (#224)

So closing this in favor of #224