Tracking TCN 0.5.0 discussions

[hdevalence]

The current state of the repository has TCN 0.4, which is effectively just assigning a version number to the CEN protocol. This allows people to make forward progress on applications. But ideally, we'd like to have one common protocol to allow interoperable contact tracing applications, combining design elements from other proposals, e.g., DP-3T, FMCEN, and now the Apple/Google protocol.