Add MLS key management interface to SFrameTransform

[bifurcation]

RFC 9605 defines a way to set SFrame KIDs and keys from a secret exported from MLS. This scheme is nice because it allows the application to set one secret and have the per-sender keys automatically derived for each possible sender.

(This is honestly more of a nice-to-have, but could help tie together the MLS+SFrame E2EE story in a world where MLS might be implemented in the browser.)

[youennf]

Agreed, let's do this once we have a good SFrameTransform basic object.