Do not log the registration code in the RegistrationState

[lslezak]

Problem

• The hot fix in Do not log the registration code #3167 is too aggressive, to avoid logging the registration code it removes the logged message completely. That might make debugging more complicated in some cases.

Solution

• Implement the fmt::Debug trait for the RegistrationState structure so it replaces the configured registration code if present with "[FILTERED]" string so the code is not logged.

Testing

• Added a new unit test
• Example output:

RegistrationState { product: "SLES", version: "16.1", code: Some("[FILTERED]"), email: None, url: None, addons: [] }