Add JSON output to `user:report`

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Is your feature request related to a problem? Please describe.
It would be useful to have user:report capable to output the results in JSON format for easier parsing and ingestion by a data processing script; I'm mainly thinking to an application monitoring use case here, in which the total number of users is tracked and plotted over time.

Describe the solution you'd like
Add an option switch --output=json to user:report, similar to other occ commands.

Describe alternatives you've considered
The app user_usage_report can report some of the information provided by user:report and it can do that in JSON format. However it lacks some useful variable, such as the user backend and the status (enabled / disabled).

Another option would be to enrich the endpoint in serverinfo with the total number of users, enabled / disabled and by backend.

Additional context

Start by locating the existing user:report OCC command and compare its options and output with other OCC commands supporting --output=json. Review the user_usage_report app and the serverinfo endpoint mentioned as alternatives to understand the required user, backend, and enabled/disabled status fields. Done means user:report --output=json emits those results in a parseable JSON format.