[FEATURE REQ] Expose immutableID property during User creation/reading

**Is your feature request related to a problem? Please describe.**
In order to use Azure/AzureAD with a federated, external IDP (https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-saml-protocol-reference) it is important to define a users immutable ID during creation so the external IDP can use it. This parameter is already present in the `UserCreateParametersInner` class but not exposed to be set in the fluent API.

**Describe the solution you'd like**
The fluent API should expose this parameter so it can be set during user creation (and also when retrieving the users information again from Azure.

**Describe alternatives you've considered**
At the moment our workaround is to set this private property via reflection magic which works (example is in Kotlin):
```kotlin
fun WithCreate.withThisImmutableId(immutableId: String): WithCreate {
val user = this as ActiveDirectoryUser

val paramField = user.javaClass.getDeclaredField("createParameters")
paramField.isAccessible = true
val innerParams = paramField.get(user) as UserCreateParametersInner
innerParams.withImmutableId(immutableId)

return this
}
```
extraction can be performed similar also via reflection:
```kotlin
val getInnerAsyncMethod = adUser.javaClass.getDeclaredMethod("getInnerAsync")
getInnerAsyncMethod.isAccessible = true
@Suppress("UNCHECKED_CAST")
val obInner = getInnerAsyncMethod.invoke(adUser) as Observable
val innerUser = obInner.toBlocking().first()

return innerUser.immutableId()
```

Do you accept PRs regarding this issue? We could supply an extension PR to the fluent API if you like.

Beginnen Sie mit UserCreateParametersInner und der von ActiveDirectoryUser verwendeten Fluent-API-Implementierung und untersuchen Sie anschließend die im Issue gezeigten Pfade createParameters und getInnerAsync. Die Aufgabe ist abgeschlossen, wenn immutableId über die Fluent API gesetzt und aus dem zurückgegebenen Benutzer abgerufen werden kann, ohne Reflection.

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