Custom Tasks: Allow use of workspaces

[GregDritschler]

Feature request

This is a follow-on to #3463.

Custom tasks currently cannot be passed workspaces. This may limit their usefulness since workspaces are a common way for sharing information.

Use case

This is needed to complete the work for https://github.com/tektoncd/community/blob/master/teps/0002-custom-tasks.md

[GregDritschler]

I will work on this after #3463 is merged.