Consider making conhost internal interfaces derive form IUnknown, be COM-like

[DHowett]

This would solve a bunch of our internal ownership issues in a way that's slightly better than "sometimes it has a shared_ptr, but the class doesn't know".

Depending on which runtimeclass implementation we use, we could also get weak references. We could also just use non-owning pointers where there would be a reference cycle.

Consider:

• IUiaData -- this would also help clear up ownership in Terminal between TermControl and TermControlFrameworkAutomationPeer
• IRenderData
• IControlAccessibilityInfo
• IUiaEventDispatcher
• ITerminalApi and ITerminalInput?
• Many of the things in interactivity- IConsoleWindow, IHighDpiApi, IConsoleControl, IConsoleInputThread, IWindowMetrics, etc.
• IRenderEngine IRenderTarget IRenderThread (let the renderer keep itself alive if necessary, fix some ownership issues with render engines and render base)