INTLY-1100 Inhibit duplicate alerts with different severity

[aidenkeating]

An alert could escalate it's severity over time. For example, after one minute of a Pod not existing an Alert with the Warning severity could trigger, after five minutes an Alert with the Critical severity would trigger. We shouldn't have both of these active in AlertManager at the same time, instead we should inhibit the less severe alert when the escalated alert triggers.

Verification:

• Deploy this branch of the operator
• The alertmanager-application-monitoring secret should contain the inhibit_rules featured in this commit.
• Go to the PrometheusRule created in the namespace
• It should contain a DeadMansSwitch rule with severity None
• Change this to severity Warning
• Duplicate it with a severity Critical
• You should now see two Alerts in the Prometheus dashboard
• You should see one triggered Alert in AlertManager with the higher severity (Critical).

[aidenkeating]

@pb82 @david-martin Mind taking a look?

[pb82]

:eyes: