Common Expression Language Custom Tasks

We propose using [`Custom Tasks`](https://github.com/tektoncd/pipeline/blob/master/docs/pipelines.md#using-custom-tasks)
to provide [Common Expression Language (CEL)](https://github.com/google/cel-spec) support in Tekton Pipelines
[without adding it to the API surface]((https://github.com/tektoncd/community/blob/master/design-principles.md)).
Users would define a [`Run`](https://github.com/tektoncd/pipeline/blob/master/docs/runs.md) type with
`apiVersion: cel.tekton.dev/v1alpha1` and `kind: CEL`. The `Run` would take the CEL expressions to be evaluated
as `Parameters` and would produce the `Results` of the evaluation with the same names as the `Parameters`. The `Results`
could be used in `WhenExpressions` or `Parameters` in subsequent `Tasks`.