fix: Prevent creating multiple endpoint ids

[cshfang]

Description of changes

Currently, when an endpoint is created by updateEndpoint, it is not cached until a full network round trip to the service. This results in a large window during which additional updateEndpoint calls will create new endpoint ids due to none being found in the cache.

This PR adds a utility which will hold created endpoint ids in memory until it is successfully cached or if the endpoint updating has failed.

Issue #, if available

#13272

Description of how you validated changes

• yarn test
• Added thrown error expectation to original code and asserted the resulting behavior of the updateEndpoint function is the same (async void which throws uncaught)
• Tested in sample app and validated that network race condition was mitigated - the updateEndpoint calls triggered by initializePushNotifications and an adjacent identifyUser call were no longer creating separate endpoints.

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.