feat: CRP-2889 add encrypted chat example

[altkdf]

No description provided.

[altkdf]

Currently remaining TODOs for the Rust backend:

• Testing workflow
• Implement vetKey API for caching and eventually use encrypted maps
• Implement vetKey resharing (API + storage datastructures)
• Implement the removal of expired messages and cache: if a vetKey epoch expires, we need to remove all associated messages, cache, and resharings. This can be done by a timer job.

Frontend:

• Key rotation.
• Key resharing.
• Cleanup and checking that everything works.
• Maybe some UI improvements.
• Introduce reusable abstractions for ratchet/rotation.

[altkdf]

The todo_remove variables will be removed once the example works end-to-end.

[fspreiss]

Approving this, so that it can be merged. Added a disclaimer to the README that this is an unfinished prototype.