Integrate metric-config-parser into outcomes/segments parsing

[mikewilli]

          And relatedly, most of the contents of this file could probably be replaced with `metric-config-parser` (though it will likely require some updates first). Again, comment is meant as a reminder to myself for later.

Originally posted by @mikewilli in https://github.com/mozilla/experimenter/pull/11243#discussion_r1736498873

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