Update CMOS to use the alert-overrider approach

Now the licensing for https://github.com/lablabs/prometheus-alert-overrider has been sorted we should integrate this pre-processing approach into CMOS.

See CMOS-23 for previous approach, we should update to the new one along with good examples of how it can be used. This will likely require some triggers during Prometheus startup, i.e. in it entrypoint.

Provide documentation and testing as well.

For testing we should have some simple verification tests we can run easily with known input and expected output to confirm it is working ok (this will be useful also if we take an update to it to verify no change in behaviour, etc.). It then feeds into the integration tests where we use the whole stack with configurable rules.