Introduce step field for KeptnMetric

[rakshitgondwal]

Goal

To imply aggregation functions, we would need a spec.range.step field.
This field can be defined as the Query resolution step width in duration format.

spec:
  range:
    step: <duration in string>

We already have a validating webhook that checks if range.interval can be parsed by https://pkg.go.dev/time#ParseDuration.
Modify the webhook so that checks the same for range.step too.