Add passive client tests for proposals

[mulmarta]

Covers:

Handling a Commit containing...

• Add
• Update
• Remove
• PreSharedKey
  • ... with a resumption PSK
  • ... with an external PSK
• GroupContextExtensions
• ... included by value
• ... included by reference

[mulmarta]

The PSKs are now in the JSON. I also added the ratchet tree to enable joining using tree outside extension.

[bifurcation]

Verified in MLSpp