Turn `ARTDevicePushDetails.state` into an enum

[lawrence-forooghian]

According to PCP4 and the documentation for the "get details from a registered device" REST API operation, this property can only take one of three values: active, failing, failed.

(There’s a discrepancy between the two documents – failed is missing in one of them – which we should resolve before working on this. See https://github.com/ably/specification/issues/24.)

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