Add strong typing for PipelineResources

[bobcatfish]

Expected Behavior

PipelineResources should be able to define their params in their spec with types, just like other CRDs, instead of being limited to just key/value. For example:

apiVersion: tekton.dev/v1alpha1
kind: PipelineResource
metadata:
  name: my-foo
spec:
  type: foo 
  url: https://github.com/vdemeester/foo.git
  banana: coffee

(@vdemeester @pmorie I'm assuming we can't get around keeping the type field if we want to add this in a backward compatible manner?)

Actual Behavior

The current model of PipelineResource is a type (string) and a list of params (key:string, value:string). This has some limitations:

• There is no way for a consumer of the api, to know what params should be there for a given type (as opposed to an actual type where these would be fields).
• Complex types used to configure resources must be projected onto a set of key-value pairs
  It is not well defined how existing parameter API would be compatible with API conversions
• Users of k8s expect conversion behavior from first-class APIs, but not from key-value pairs (as resource parameters are currently defined)

The above example PipelineResource would today need to be:

apiVersion: tekton.dev/v1alpha1
kind: PipelineResource
metadata:
  name: my-foo
spec:
  type: git
  params:
    - name: url
      value: https://github.com/vdemeester/foo.git
    - name: banana
      value: coffee

Additional Info

This must first be implemented in a backwards compatible way, which would mean supporting both the key/value fields and the new typed fields for one release.

(Most of this content comes from #238 and is @vdemeester / @pmorie 's words :D)

[pmorie]

I'm wondering if we should make any changes here before we agree on the extension model. If we have agreement on the goal state first, we can avoid multiple API changes for users. Also, is backward compatibility important here? If so we should take that into account in the resource extension design.

[dibyom]

The new pipeline resources design can help here: #1673 ?

[bobcatfish]

Since we're completely revisiting PipelineResources via #1673 I'm going to close this but plz re-open if you want to discuss more!