jobs, sql, ui: Create AutoCreateStats job type

With #34279, enabling the cluster setting
`sql.stats.experimental_automatic_collection.enabled` has the potential
to create many CreateStats jobs, which can cause the Jobs view on the
AdminUI to become cluttered.

This commit creates a new `AutoCreateStats` job type for these auto-created
CreateStats jobs, so that users are able to still see their own manual runs
of CREATE STATISTICS, via the pre-existing `CreateStats` type.

![jobs-auto-create-stats](https://user-images.githubusercontent.com/3051672/54212467-5cea2c80-44b9-11e9-9c11-db749814f019.gif)

Release note (admin ui change): AutoCreateStats type added to
Jobs page to filter automatic statistics jobs.

Fixes #34377.