Add support for Validation (Non-Privileged)

[dansiegel]

Description

There are a number of scenarios where we may want to validate that the system is in a good state or find out what needs to be fixed without actually fixing anything. We should have an option to execute something like uno-check validate to get a summary of what is good/bad... and perhaps a uno-check validate -json to output json content that could help tooling running Uno Check from a process to surface what the CLI tool reports back.