Add support for extension attributes

todo: add all 15 extensionAttributes as selectable attributes in the sync form, and manage special mapping for these. They are nested within the user object.

--

These are things like:

extensionAttribute1
extensionAttribute2
...
extensionAttribute15

These are available on the user object as:

onPremisesExtensionAttributes

Example:

{
  "onPremisesExtensionAttributes": {
    "extensionAttribute1": "Marketing",
    "extensionAttribute2": "VIP"
  }
}