Add native support for CronJobs

[nicolaferraro]

To complete the Camel K "serverless experience", a simple addition would be the support for Kubernetes CronJob out of the box.

A user running an integration like:

from('timer:tick?period=1m').to('...')

Can have it materialized as cronjob.
In practice, a pod will be created by Kubernetes only when it's time to run the integration and there's no pod running between executions.

This rocks in combination with Quarkus.

There will be periods of the timer that cannot be supported (sub minute, or periods that cannot be expressed by a cron), but that should be fine, we fallback to normal implementation.

Equivalently, we can also add:

from('quartz:tick?cron=* 0/1 * * ? *').to('...')

The translation is easier, even if quartz supports 6-7 fields in the cron expression, while Kubernetes only 5 (no seconds, no years).

This requires some changes also at runtime level.

[lburgazzoli]

On the runtime side, we can use a trick similar to the one we use for knative-source, like have a cron loader that replaces the from definition with a simple time that fires immediately and only once.

Do you mind opening an issue on runtime side too?

[nicolaferraro]

Opening.. I've sketched a solution using a different trick (customizer), so that it can be used also when you need a knative-source loader.. Will try to publish soon, so we can iterate on that