fix: Wait for endpoint creation to identify user

[cshfang]

Description of changes

When the Push Notification category is first creating a Pinpoint endpoint upon receiving a device token, it is possible for an identifyUser call to be attempting to update the endpoint to be resolved first over the network. This is troublesome for at least two known reasons:

1. The identifyUser call could be attempting to set the OptOut property to 'NONE' and if it is resolved first by the service, the service will reject the request due to the Address not having yet been set.
2. The identifyUser call could contain userId or demographic information and if it resolved first by the service, the request made by the library to register the device token would contain default values which would unintentionally overwrite the ones set by identifyUser

This PR mitigates this by having identifyUser calls (for Push Notification channels) await the resolution of device registration during endpoint creation (i.e. this blocking does not occur during endpoint updates).

Issue #, if available

#13174
#13272

Description of how you validated changes

• Added unit tests for added use cases
• yarn test
• Tested in a sample React Native application to assert identifyUser calls were blocked on the endpoint creation triggered by device registration but not on subsequent updates

Checklist

• PR description included
• yarn test passes
• Tests are changed or added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.