feat(ads-client): roundtrip cap key for spoc impression callbacks (AC-125)

[jonesetc]

Add roundtripping of cap_key in sposored content impression callback urls. This value is currently just discarded, but in the future will be used to keep a rolling count of impressions in the last 24 hours on device to make sure sponsored content is not being shown too often.

This does not change any internal or external APIs, and does not require an entry in the changelog

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