Let a user specify a "remote" file for CEL

[chmouel]

Instead of having to specify the full CEL expression into the annotations (which doesn't scale) we could have a syntax that would reference a remote reference to a file containing the full CEL.

This would allow to write and share long CEL filtering across pipelirun or repository.

example:

 pipelinesascode.tekton.dev/on-cel-file: .tekton/file.cel # resolve .tekton/file.cel at the same revision time where the event is triggered.

 pipelinesascode.tekton.dev/on-cel-file: https://raw.github.com/foo/bar/.tekton/file.cel # http get of the file