Make deviceDisplayName optional in serialized DeviceRegistration

[Whathecode]

When DeviceRegistration.deviceDisplayName was introduced (#350), there was some discussion on whether or not it should be optional. I landed on making it optional, but, for some reason still kept the @Required attribute defined on it, which causes it to always be serialized, even if it is null.

It sounds like null makes a fine default field, which when the name isn't set, shouldn't be serialized.

The fact that it was set to @Required caused some confusion in another PR which introduced a new DeviceRegistration field (#484). If it's not needed, it should probably be removed. Doing so will require a version bump of APIs and adding an application service migration to the services which return this type (for backwards compatibility of older clients). This task should thus be bundled with an API upgrade.

[Whathecode]

@yuanchen233 I created this new task for our discovery that deviceDisplayName can likely be optional in serialization. Since we are targeting a new API release soonish, it may make sense to change this as part of the next version bump in the API.

It will also be a good exercise to make use of the API migration functionality. You can have a look at DeploymentServiceApiMigrator.