[Experimental] Object reference resolver to support non-Addressable objects

**Description**

This proposal is about extending the KReference resolver to support non-addressable objects (no `status.address` field). For those objects, we propose to ~delegate the reference resolution to an external service which upon receiving a KReference object replies with a resolved URI or an "not supported" error~ allow people to define a mapping between "Kind" and "URL-Template", where the URL-Template can use fields from the Object being referenced.

**Use Case**

The driving use case is this one: for instance, as a user I want to be able to trigger a batch job every day by having a PingSource CR targeting a batch job definition:

**IMPORTANT NOTE: this is just an example! The source can be anything (Github, Kafka, you name it) and JobDefinitition can also be anything that is not Addressable.**

```
apiVersion: my.batch.job/v1alpha1
kind: JobDefinition
metadata:
name: pi
spec:
template:
spec:
containers:
- name: pi
image: perl
command: ["perl", "-Mbignum=bpi", "-wle", "print bpi(2000)"]
restartPolicy: Never
backoffLimit: 4
---
apiVersion: sources.knative.dev/v1
kind: PingSource
metadata:
name: trigger-job
spec:
schedule: "@daily"
sink:
ref:
apiVersion: my.batch.job/v1alpha1
kind: JobDefinition
name: pi
```

**Exit Criteria**
Users are now able to reference non-addressable object

**Experimental flag name**: `kreference-custom-resolvers`

**Experimental feature stages plan**

Below the proposed plan for the feature stages (this list implicitly includes the requirements defined in the process)

* Alpha:
* [x] Extend `URIFromObjectReference` to support custom resolvers in knative/pkg
* [x] Add kreference-mapping experimental flag
* [x] Add a new configmap (kreference-mapping.yaml) to configure mappings between "Kind" and "URL-Template"
* [x] Implement kind -> URL mapping URI resolver
* [x] PingSource uses the new custom resolver
* [ ] Track reference
* [ ] All controllers relying on sink resolver uses the new custom resolver
* [ ] User documentation
* Beta graduation as soon as 1 release after the inception
* Beta:
* [ ] User documentation stabilization and improvements
* [ ] More e2e tests
* [ ] Add conformance tests
* Stable graduation as as soon as 2 releases after the Beta graduation
* Stable:
* [ ] Add the requirement to the spec

**Affected WG**

* Eventing WG
* Networking WG

Start by reviewing URIFromObjectReference in knative/pkg and the kreference-mapping.yaml configuration, then check how PingSource uses the custom resolver. Compare the unchecked tracking, controller, documentation, e2e, conformance, and spec items with the current implementation; done means the remaining acceptance criteria are implemented and verified.

Written by the indexing model from the issue text.