Feature Request: add an optional not_equal field to inhibit_rules

The alerts in AlertManager may have severity labels to route alerts to the right receivers. It should also be possible to inhibit an alert with a lower severity level if an alert with a higher severity level and otherwise equal labels is present, e.g.:

inhibit_rules:
  - source_match:
      severity: 'critical'
    target_match:
      severity: 'warning'
    not_equal: ['severity']

It does not work to simply put equal: ['alertname'] because the alert may consist of other labels that should be considered. Currently, it is possible to specify the labels that should be equal, but it should also be possible to specify a list of labels that should not be equal, so that the other labels can be compared for equality without necessarily knowing the label names in advance.

Start with AlertManager's inhibit_rules handling and the existing equal-label comparison. Trace how source_match and target_match are evaluated, then verify the YAML example and confirm that not_equal labels are excluded from equality comparison while all other labels remain comparable.