Centralize warning reporting

[RaulBSanchez]

Context

This was done as work for the bug #75

Changes

1. I routed scheduler warnings through on_discovery_warning
   This was done to use the same flow as the discovery warnings
2. I had to wrap the scheduler warning strings into a DiscoveryWarning object
   I got an error after changing reporter.on_scheduler_warning(w) to on_discovery_warning(w)
   It was trying to pass a string but need an object
3. Added Scheduler to on_discovery_warning to handle this error
   DiscoveryWarningKind::Scheduler` not covered
4. Used AI for this part - Added handling for DiscoveryWarningKind::Scheduler in the text reporter. This lets the scheduler warnings print correctly

Test plan

Cargo Test
They all passed

[thejchap]

Thanks @RaulBSanchez ! Made some minor tweaks

[RaulBSanchez]

Sweet - thanks @thejchap will try to make sense of them! Thanks again for letting me work on this project.