Add portName to Operator and Webhooks for Consistency

[ramasai1]

The metricServer prometheus section in the KEDA chart provides a portName field which is the name of the port in both the deployment and the service monitor. This is not the case for the operator and admission webhooks. There's not much consistency in how the port name for the deployment is chosen vs how the service/pod monitor objects use said port.

This PR unifies the interface by allowing each component to define a portName and makes sure that this is the name of the HTTP port being exposed in the deployment + this is the port being used by the service/service monitor/pod monitor.

#647 mentions the same thing but I chose to take a different route.

This may cause a breaking change because of serviceMonitor.port not getting respected anymore.

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Commits are signed with Developer Certificate of Origin (DCO - learn more)
• README is updated with new configuration values (if applicable) learn more
• A PR is opened to update KEDA core (repo) (if applicable, ie. when deployment manifests are modified) -- NOT APPLICABLE.

[ramasai1]

@JorTurFer could I please get a review here?

[zroubalik]

We shouldn't introduce breaking changes, can we support both approaches?