Add PostureIdentity fields to Device #48
Description
The [device type](https://tailscale.com/api#model/device_ includes information about posture identity, which is not currently supported in this Go API wrapper. Let's add it.
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The [device type](https://tailscale.com/api#model/device_ includes information about posture identity, which is not currently supported in this Go API wrapper. Let's add it.