[celeval] CELEval Custom Tasks CRD

We introduced CEL Custom Tasks to experiment with using an expression
language with Tekton Pipelines.

Given feedback from the past several months of usage, we have identified
three main current challenges:
- CEL custom tasks do not take variables for the CEL environment.
As such, users cannot evaluate CEL expressions given specific variables
or in specific context. For example, as described in
tektoncd#716 and
tektoncd/community#403, a user needed to
declare runtime storage variables in the CEL environment.
- CEL custom tasks are not a CRD thus making them unreusable across
different Runs and PipelineRuns. Read more in
tektoncd/community#314 (review).
- CEL custom tasks take the CEL expressions through Parameters which is
misleading to some users. Read more in
tektoncd/community#314 (review).

To address the above challenges, this change introduces a CRD for CEL
Custom Tasks, which takes CEL expressions and CEL environment variables.
With this change:
- CEL custom tasks now take variables for the CEL environment
- CEL custom tasks are reusable across different Runs and PipelineRuns
- CEL custom tasks take expressions through its own field

Closes tektoncd/community#403
Closes tektoncd#716