Rename "signature" to "secret"

[milosmns]

Provide a brief summary of the planned work

All user properties named "signature" should be renamed to "secret" to better reflect the use.

Detailed overview

Signatures were used in the initial versions of this service (in Python), but they are now not used anymore. All user and credential properties called signature should be removed and replaced with secret.

Hint/location:

com/appifyhub/monolith/features/user/storage/model/UserDbm.kt:29

List the acceptance criteria

• All signature properties are replaced with secret
• All function names and arguments are updated to reflect that change
• All tests are updated and passing
• Documentation is updated to reflect the change in the network layer

Include any additional notes (optional)

Related issues:

• Redact secrets in network responses #171