Add support for CustomRuns

Feature request

CustomRuns are first class Tekton objects (https://tekton.dev/docs/pipelines/customruns/). With support for CustomRuns being added to Tekton Chains and Tekton Results, extending the pruner to support CustomRuns is a natural extension.

Use case

With CustomRuns support added to Tekton Results, users can opt in for pruning objects from the cluster after being stored. If users do not use the Tekton Results pruner, but opted to use the Tekton Pruner instead, there is no way to have the Tekton Results pruner for one object (CustomRuns), but not for other (TaskRuns, PipelineRuns), so to give the users single way to manage pruning, the Tekton Pruner must be extended to support CustomRuns.