Add agent-child-processes-status blueprint

It was added for Juno, but didn't fully made it before
the feature freeze. Re-adding, with updated template
and implementation details.

blueprint agent-child-processes-status

Change-Id: I5701d5b8858c7d6dc6c8d3f3e35379c62b2699c9
Merge "Add agent-child-processes-status blueprint"