finish adding ref and selector fields for all Grafana OnCall resources

[joeyorlando]

Description of your changes

Adds all missing selector and ref fields for the following Grafana OnCall Crossplane resources:

• oncall.grafana.crossplane.io.EscalationChain
• oncall.grafana.crossplane.io.Escalation
• oncall.grafana.crossplane.io.Integration
• oncall.grafana.crossplane.io.OnCallShift
• oncall.grafana.crossplane.io.OutgoingWebhook
• oncall.grafana.crossplane.io.Route
• oncall.grafana.crossplane.io.Schedule

Closes grafana/oncall#4411

I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable test to ensure this PR is ready for review.

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[joeyorlando]

@MasterExecuson see this PR.

One thing to note is that for any related object which is only represented by a Terraform datasource (as opposed to a resource), Crossplane doesn't allow us to specify refs/selectors for these. Terraform datasources are not translated to Crossplane resources unfortunately.

The workaround they have is this, basically using the Terraform provider for Crossplane to use the datasources directly.

Once we get v0.17.0 of this provider published, let me know if you're able to get this working with the linked documentation.

[julienduchesne]

LGTM!