Fix paginated contact list deserialization

[iPlexor]

Why?

Fixes Issue #160 ContactsClient.List() returns JSON that can't be deserialized. Specifically, the next object:

{
    "type": "list",
    "data": [ ],
    "pages": {
        "type": "pages",
        "next": {
            "page": 2,
            "starting_after": "WzE1ODk0NjU0NTkwMDAsIjVjOTBjYTg0NjQxNjBjMTc2OTFmOThhZiIsMl0="
        },
        "page": 1,
        "per_page": 10,
        "total_pages": 27
    }
}

How?

This PR adds Next.cs and uses it instead of a string for the type of next in Pages.cs

[iPlexor]

It looks like the UserConversationsClient.List() and AdminConversationsClient.List() responses actually return next as a string, meaning this PR will break deserialization of those lists.

Looks like the API is serving two different models of Pages, which is incompatible with the shared Pages data structure of intercom-dotnet.