In order to implement Service Accounts (#2597), a service account corresponding to an extension needs to be created on extension registration. As part of this process, the service account should be stored in the internal user storage alongside the internal user accounts.

The general implementation process will require adding code to place a created service account inside the internal users storage. This will require changes in the Security plugin. Specifically changes will be required in the UserService class so that when a service account is created it is stored in the configuration repository for the internal users. The configuration will also need to be saved and restarted so that the state is consistent across all nodes.

Completion of this issue will look like a PR which adds to the UserService class. The code should add a service account to the internal users storage if a matching account does not already exist. The PR should also include tests confirming that the service account is added upon creation.