Make `pgagroal-admin` provide JSON output

[fluca1978]

Since pgagroal-cli now has a complete JSON output support, it is time to provide also pgagroal-admin with JSON support. The implementation should be simple, since no management protocol is involved, and the idea is that every command can directly output a JSON with all the details about the action. The most notably use case is for user ls that has to provide a list with all the users.

[T1t4m1un]

Could I take on this ticket?~

[fluca1978]

@T1t4m1un yes, sure, thanks!