Experiment Variation - Return optional instead of control

[selanthiraiyan]

Related to - woocommerce/woocommerce-ios#8806

Changes

• ExPlat returns optional instead of assuming control as experiment variation. This lets the client know that there is no variation for an experiment.
• Make Variation confirm to codable.

Why

In WooCommerce iOS, we will cache the variations for the logged-out experiments. We are doing this to try to remove the high crossovers in Abacus that we face for logged-out experiments.

Choosing this solution after exploring other options. More background in woocommerce/woocommerce-ios#8788

Testing

• CI should pass successfully.
• Test [ABTesting] Cache variations of logged out context experiments woocommerce/woocommerce-ios#8806

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• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[itsmeichigo]

LGTM