Mount resource ConfigMaps as subPaths instead of volumes

[astefanutti]

Following up the discussion from apache/camel-k-runtime#593, it may be worth to explore mounting resources ConfigMap using SubPath. As opposed to mounting them as volumes, it's possible to project keys into an existing directory, which could alleviate the need to generate unique directory paths for each resource, and ease the work for users to reference them.

The only side-effect of using SubPath is that updates to the ConfigMaps are not automatically projected into the containers. But I don't think that's an issue for Camel K.

It hasn't been leveraged as SubPath was not historically supported for Knative service deployments. However, it seems that limitation has been removed in Knative: knative/serving#3926.