Support returned secrets in encrypted form

[peacekeeper]

In certain cases, the DID Registrar returns private keys and other secrets to a client (see didState.secret).

Should we support that in an encrypted form?

See #6 (review)

[alenhorvat]

What about the following statement:

For backwards compatibility, DID Registrar supports a "secret mode", where the private keys are created by the DID Registrar driver and are returned to the user. For security reasons, this approach is NOT RECOMMENDED. If the secret client mode is used, the private key MUST be encrypted.

The HTTP response header MUST contain
Cache-Control=no-store
Pragma=no-cache