.NET: [BackagroundResponses] Allow ChatClientAgent to work with non-ChatClientAgent continuation tokens

[SergeyMenshykh]

Context:
As part of addressing this issue: #1762, a new continuation token type specific to ChatClientAgent will be added. The continuation token will wrap a continuation token returned by IChatClient and will allow ChatClientAgent to add additional data that will be accessed during the stream resumption phase, together with the original continuation token which will be sent to IChatClient for resumption.

ToDo:
Update ChatClientAgent to accept continuation tokens of the underlying IChatClient type, to enable scenarios such as push notifications. In these scenarios, a host will mint (based on information from the push notification payload) a continuation token that the underlying IChatClient supports, and ChatClientAgent will pass it through to the chat client.