Allow for global defaults for trigger configuration

Proposal

We want to be able to to specify global defaults for trigger parameters, like the Endpoint for the Prometheus trigger. We have 100s of services using KEDA and having their scaled objects individually specify the endpoint introduces a large surface area for error.

There are a couple of ways I think we can do this:

• Create a ClusterTriggerConfiguration CRD to surface the ability to set global configuration options for a Scaler. For example, this can include endpoints, any required headers to be injected in all requests, etc
• Extend ClusterTriggerAuthentication to support adding endpoints for Prometheus
• Extend Prometheus Scaler to support reading from environment variables.

Use-Case

• By specifying this as a global default, we can update and manage these defaults from a single deployment, improving response if we need to, for example, change this endpoint for an environment.

Is this a feature you are interested in implementing yourself?

Yes

Anything else?

Once again, happy to implement this myself, but I'd like to discuss how to do it, either via a config map or via environment variables.