Chains support for CustomRuns

Not sure if this would be considered a bug or feature request.

Currently, Chains will not process on pipelines that contain CustomRuns. If this is not supported by design, I could not find any documentation on the subject. Ideally, this is either supported or documented.

The pipelines I am working with use CustomRuns to set github commit statuses. After implementing Tekton Chains, my pipelines without these CustomRuns are processed properly, but those with them produce the error Unable to get reconciled status of taskrun <customrun task name> within pipelinerun in the chains-controller.

Notes for consideration:
SLSA L2 requires isolated builds. So I do not think CustomRuns would satisfy this requirement due to logic typically running in the same pod. However, I think I could make the argument that in my use case, the CustomRun is not part of the "build pipeline".

SLSA L1, on the other hand, requires provenance and I think Chains should be able to provide that for all types of Runs.

Start with the chains-controller handling of reconciled TaskRun status and reproduce the reported error using a PipelineRun containing a CustomRun. Determine whether CustomRuns should be processed for provenance, or whether their unsupported status needs documentation; done means the intended behavior is implemented or clearly documented for the described SLSA use cases.