Cannot manage encrypted subscriptions via SDK due to missing interfaces.

[jasonjoh]

Describe the bug
Attempting to use AddPublicEncryptionCertificate on a Subscription results in an error:

'Subscription' does not contain a definition for 'AddPublicEncryptionCertificate' 
and the best extension method overload 
'IEncryptableSubscriptionExtensions.AddPublicEncryptionCertificate(IEncryptableSubscription, X509Certificate2)' 
requires a receiver of type 'Microsoft.Graph.IEncryptableSubscription'

Attempting to use DecryptAsync method on ChangeNotification.EncryptedContent results in an error:

'ChangeNotificationEncryptedContent' does not contain a definition for 'DecryptAsync' 
and the best extension method overload 'IDecryptableContentExtensions.DecryptAsync<ChatMessage>
(IDecryptableContent, Func<string, string, Task<X509Certificate2>>)' requires a 
receiver of type 'Microsoft.Graph.IDecryptableContent'

Attempting to use AreTokensValid on a ChangeNotificationCollection results in an error:

'ChangeNotificationCollection' does not contain a definition for 'AreTokensValid' and 
no accessible extension method 'AreTokensValid' accepting a first argument of type 
'ChangeNotificationCollection' could be found (are you missing a using directive or 
an assembly reference?)

Expected behavior
These methods should be available. This is a breaking change from v4 SDK.

[baywet]

@andrueastman if we want to restore that we might want to switch classes generation (at least for models) to be partial. I'm a bit reticent to that approach since it'd only work for dotnet.
The alternatives being:

• use of reflection (not great)
• extension methods in the service libraries (only works for dotnet as well)
• duplicating the code across service libraries (probably the least of all evils)

Thoughts?

[andrueastman]

@baywet Given that the ChangeNotification models are not generated by Kiota as they are not directly referenced by any endpoint(the openApi metadata would need to reference them using callbacks and then Kiota detects the models in them)

I think the last option would be the best here as the existing webhook models already use that approach.

[baywet]

ah I missed that aspect, so they are effectively defined in core I guess?