Add support for the Safe Extensions framework

[rohanmahy]

Description of feature:

Please consider adding support for the Safe Extensions Framework, described in Section 2 of draft-ietf-mls-extensions.

Use case:

This would allow users of the stack the ability to add safe KeyPackage, LeafNode, GroupContext, and GroupInfo extensions, safe use of secrets and signatures, safe proposal types, safe wire format types, and (if PR#29 is accepted) safe usage of AAD.

[tomleavy]

Thanks for the heads up @rohanmahy

@mulmarta you are probably more familiar with this than I am, how difficult do you think it is to add?

[mulmarta]

It won't be too difficult, I think. Mostly just adding more API to what we already have.

I think maybe we want to put it behind a feature flag, in order not to complicate the basic API?