Add Purchases.shared.attribution.setAmplitudeUserId to the SDK

[alpennec]

Hello,

Can you update the SDK to add a method to set the UserID/DeviceID instead of having to use the reserved attribute $amplitudeDeviceId or $amplitudeUserId ourselves?

https://www.revenuecat.com/docs/integrations/third-party-integrations/amplitude

Thanks,
Axel

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