Add UserManagement entity

[mshima]

UserManagement entity can be used to generate a ui for user management instead of the custom one.

It's possible to test it using the following jdl:

application {
  config {
    baseName usermanagement
  }
  entities *
}

@SkipClient(false)
entity UserManagement

• it can be used to regenerate the user-management components using new standards.
• can be used by blueprints like Support for the Vaadin UI Framework #24794.
• could be considered for v9 to make User customizable.
• authority is converted to list of strings, so quirks are required to make it work.

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