Add ChatSessionInputState

[mjbvz]

Adds the concept of a managed ChatSessionInputState object. This object is how extension can interact with the chat session input box

@TylerLeonhardt @DonJayamanne Can you please review the api and let me know what you think. This PR is just the api changes for now. Here's a few example flows:

New blank chat editor

1. VS Code invoke provideChatSessionInputState on your controller.
2. The returned options are shown to the user

Use presses submit on blank chat editor

1. The previously returned input state is passed to newChatSessionItemHandler to create the chat session item
2. It is then also passed to provideChatSessionContent

Use reloads with a chat editor visible

1. We call provideChatSessionInputState again so that extensions can still provide custom options. However we also pass in the previously selected options so that you can use these if you need

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