Migrate outcomes and segments dataclasses to nimbus-schemas package

[mikewilli]

          No action required here, but more as a reminder to myself... Perhaps later we should consider whether this (and the related dataclasses in `outcomes/__init__.py`) would be better placed in nimbus-schemas.

Originally posted by @mikewilli in https://github.com/mozilla/experimenter/pull/11243#discussion_r1736494601

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