[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.

Commencez par UserCreateParametersInner et l’implémentation de l’API fluide utilisée par ActiveDirectoryUser, puis examinez les chemins createParameters et getInnerAsync présentés dans l’issue. Le travail est terminé lorsque immutableId peut être défini via l’API fluide et récupéré depuis l’utilisateur renvoyé sans réflexion.

Rédigé par le modèle d'indexation à partir du texte de l'issue.