Join-by-Welcome active tests and supporting API/logic

[bifurcation]

This PR adds a config file welcome_join.json that covers join-by-welcome cases. It also adds some gRPC mechanism that is necessary to generate the required cases:

• Adding force_path and external_tree fields to CommitRequest, and a ratchet_tree field on CommitResponse
• Adding an installExternalPSK action that generates a PSK and installs it on clients
• Updating the active-to-passive script to capture PSKs and external trees

I have verified that this works with my working branch of MLSpp, in the sense that (a) MLSpp interoperates with itself via gRPC, and (b) the generated passive test vector passes.

Depends on #128, should probably not be reviewed until that is merged and this PR is rebased.

[mulmarta]

I verified that it works between Wickr and MLSpp.