Feature: Allow customisation of warning/critical alert levels

[vCra]

For some SLOs, we don't want high burn rate alerts to have severity critical, but instead have a different value (for example severe or p1 etc)

Allow high/low burn rate severities to be configured within an SLO

This could potentially look like the following

spec:
  alerting:
    severity:
      high: critical
      low: warning

[metalmatze]

There's a PR open for quite some time that we just need to finish and finally merge: #617

[metalmatze]

I'll close this as it's a duplicate of that PR. Feel free to comment on the PR or here again, if anything else comes up.