New setting for root context/CWD at the job level

[e3b0c442]

What happened:
Branching this off of #1455, because while it's a similar concept, I think it differs enough from that request.

I would like the ability to set at a job level a root context/current working directory which is a subfolder of the repo root, such that the job sees that folder as its SD_SOURCE_DIR. This would be complementary to both the sourcePath setting and the multi-pipeline work being done in #1455.

An example use case is a monorepo which contains API and UI for a project. I would like to be able, in the same pipeline, apply a template for the UI portion to the UI job, and a template for the API to the API job. However, unless the template specifically allows for it, I may need to modify steps to make sure I am working in the correct folder.

I understand that there are ways to work around this, but they either add unnecessary complexity to the pipeline config or require overriding job steps entirely depending on the template (which then means changes to the template need to be manually tracked).

What you expected to happen:
Add a context: sub/folder (or similar) setting to the job config and have that folder treated as the root context for that job

How to reproduce it: