project.config: add Auto-Submit label

Add a new Auto-Submit label, as part of the CL autosubmission work.
For now, usage of this label is restricted to the release-managers
group. Once fully rolled out and tested, this should be widened to
the approvers group.

For golang.org/issue/48021.

Change-Id: Iba7ae026cd639855608ba61a95692410e018d35b
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/347630
Reviewed-by: Roland Shoemaker <bracewell@google.com>