[WIP] Add support for experimentation segmentation by user attributes

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original description:

Extend the Create a New Experiment API so that when creating an experiment, you can define segmentation rules (e.g. country=US, device=mobile). Only users matching the rules should be eligible to be bucketed into the experiment.

Update the experiment assignment logic to check these rules before assigning a variant.

Add validation so experiments without valid rules are rejected.

Generate unit tests covering both matching and non-matching users.

Update API documentation to show how to define segmentation rules when creating an experiment.

Assign the pull request to johnagan to review.

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