Support Multi Step Tasks

[tlawrie]

Is your request related to a problem? Please describe.
Currently a Task maps to a container with a single execution step

Describe the solution you'd like
Tekton tasks can support multiple steps within the same container.

Describe the benefits or justification for this request
Allows for sharing the same container process namespace and storage which provides advantage to certain steps.

Additional context
Add any other context or screenshots about the feature request here.