Implement token enrollment via validate/check

[lukasmatusiewicz]

privacyIDEA v3.8 allow enrolling a new token via validate/check request.

Update the java-client to use it.

• Check if in PI Response exists an img and add it to the PIResponse object.
• Update tests.

NOTE: Don't remove the old token enrollment method.

QUESTION: Should the old method generate a debug message, that there is a new, better, and safer method that is now recommended? And we recommend updating the PI server to >=3.8 to use it?
@nilsbehlen