Expand workspace support to pipelines and pipelineruns

Expected Behavior

Workspaces should be declarable in a pipeline and bindable in a pipelinerun. Threading a workspace through multiple tasks should allow for files to be shared across those tasks (if using the correct volume source types - PVC, configmap, secret).

We may also want to explore some kind of "from" syntax - a PipelineTask could declare that it wants the workspace "from" a prior pipeline task and Tekton would be responsible for exposing whatever volume source backs that workspace to both tasks.

Actual Behavior

At the moment we don't have any of the workspace types implemented for pipelines or pipelineruns.

This is a follow-on task from #1438

[vdemeester]

@sbwsg #1866 being merged, I guess this could be closed ?

👍 yes indeed!