Update Example controller to new unified standards

[scottrigby]

We already have an example controller that consumes the GitOps Toolkit Source APIs fluxcd/source-watcher. This accompanies the Watching for source changes Flux guide.

The example is however out of date with the new Flux controller standards that are currently in the process of being rolled out (https://github.com/fluxcd/flux2/issues/1601).

We need to make the example controller more complex, to help people understand how to use pkg/runtime. Must contain in-code documentation. This is a pre-step for updating the guide.

[scottrigby]

@stefanprodan suggests that we keep source-watcher as-is, because there is value in this as an example for IoT.

Thoughts on the new example:

• should not also be a Flux specialized applier / delivery controller
• example creates configmap from .env file
• something easy that no one will use in production (for example k8s already has a configmap generator that does this)
• maybe something like Ellen Korbes' fun "useless operator" (sets active/inactive lol)