Registration expiration

[tpazderka]

When a new registration is returned from the registration_endpoint, it contains a non-zero value in client_secret_expires_at. This indicates that after this time, the secret should not be valid but it is not true. The validity of secret is not checked anywhere during the processing of the requests.
According to specifications, this should return 0 in client_secret_expires_at.

But I think it would be more useful to add the checking of the client_secret validity to the code as well as a configuration option for the validity itself.

[rohe]

I've added a function for validating a client registration which basically is checking if the client_secret_expired_at time has passed or not.

[tpazderka]

Resolved, thanks!