feat(conversations): Added "missing" functionality to get truncated threads

[wotta]

According to the following documentation there is an option to get truncated threads from the conversation API.
See the documentation here: https://developer.helpscout.com/mailbox-api/endpoints/conversations/list/

Note: If using the embed=threads parameter with this call, you will see truncated chat threads. This is by design. To view Beacon chat threads in full, call the [List Threads](https://developer.helpscout.com/mailbox-api/endpoints/conversations/threads/list/) endpoint for that conversation instead.

I updated the ConversationFilters to allow using the embed=threads logic in order to minimize the time it takes for the API to return a response. When I use the new (ConversationReques())->withThreads() the api really takes a toll before a response is actually given.