Add experiment fragment to the product metrics base schemas (which currently have experiments fragment that this is superseding)

Properties:

• enrolled: name of experiment subject is enrolled in, null if not enrolled
• assigned: name of test group / variation subject is assigned to, null if not enrolled
• subject_id: random ID for identifying a subject enrolled in experiment
• sampling_unit: unit at which sampling was done, e.g. "mw-user" or "app-install"
• other_assigned: if user is enrolled in other experiments, which test groups are they assigned to?

We would bump base schema from 1.3.1 to 1.4.0 and it would have this new

In the future we will want to do a major new version of the schema (2.0.0) and remove the experiments fragment and create new versions of streams.