Add a blind signatures extension

[sev71]

Add a blind signatures extension to address the possible collusion between backend and authority (tries to solve the issue DP-3T/documents#210).

Tried to adapt your code and naming style.

Only tested with the dev config and HSQLDB.

The authority controller test shows the protocol workflow.

If there is enough interest, I could complete the feature.

[wouterl]

I don't think we should use blind signatures for now. Given the timescale, we prefer to use standardized cryptography as much as possible.

There is indeed a residual risk of the health authority colluding with the backend. However, using blind signatures alone will not solve that risk. In the current system, the app interacts with the health backend and then immediately after with the backend you see here. To prevent collusion attacks, implementations need to also hide IP addresses and time correlations. Both of which are very hard to do.