Add count, remove functionality to systems

[StephenBrown2]

This adds a couple flags to systems: --count and --lastCheckinBefore, which will show a plain count of systems filtered, and allow filtering by lastCheckin date, respectively.

It also adds the ability to unregister/remove systems, provided the same filtering abilities as with listing systems.

[flynshue]

I totally forgot this exists. Thanks for the updates 😄

[flynshue]

@StephenBrown2 I haven't used this in a long time, would you be interested in adding some tests? I really should have added them for the start but got busy :( and should have added GHA + goreleaser to make the binaries

[StephenBrown2]

Thanks for the merge!
Sadly, I probably don't have time for tests right now, though of course they are often helpful. If I find myself adding more to it, I'll try to add tests at that point.

[StephenBrown2]

It does look like GoReleaser failed though... https://github.com/flynshue/rhsm-cli/actions/runs/10475685459/job/29012891297