Allow ApiServerSource to listen for external K8s API server

**Problem**
We would like to configure the [ApiServerSource](https://knative.dev/docs/eventing/sources/apiserversource/) to listen for events emitted by a different (external) K8s API server, not the one that the knative is installed upon.
In [this Slack thread](https://cloud-native.slack.com/archives/C04LKEZUXEE/p1681631327418339?thread_ts=1681631327.418339&cid=C04LKEZUXEE) we discussed about this request.
@evankanderson and @pierDipi suggested to add a `kubeconfigSecret` to the ApiServerSource.

**Persona**
Event consumer, System Integrator.

**Exit Criteria**
The ApiServerSource will listen for an external API server trigger the configured targets.

Start with the ApiServerSource documentation and implementation, then trace how it connects to the Kubernetes API server. Determine how the proposed kubeconfigSecret should select an external server and verify that configured targets receive its events; use the issue's external API server exit criterion as the completion check.

Written by the indexing model from the issue text.