Property placeholder assist

[igarashitm]

Please describe the feature that you want to propose

When a user is authoring a route and typing in a text field, once user types {{, show a list of properties already defined in external property files in the workspace. It might be even possible to retrieve k8s configMap/secret when in the cluster, or have a k8s CRs in the workspace.
Unlike #493 , automatically adding a new entry won't always help as there could be several places the property would come from - env, system properties, k8s configMap/secret, etc

[lordrip]

Let's start with the investigation phase.