[FR] Set alert threshold per value for more fine-grained control

We have a workflow which analyzes the output of our product and and sometimes a lower values is better, while other times a higher value is better.
https://github.com/Ultimaker/CuraEngine/blob/33790412b61f33e4f4bfe1af2512f2f621e7aff5/.github/workflows/gcodeanalyzer.yml#L181

Allow for `alert_threshold` to be set per value will give a more fine-grained control
```
{
"name": "Quantity which should not increase",
"value": 12503.752753577128,
"unit": "s"
"alert_treshold": ">110%"
},
{
"name": "Quantity which should not decrease",
"value": 182291,
"unit": "-"
"alert_threshold": "<90%"
},
```