[.NET] Add support for reporting device + asset status in connectors

[timtay-microsoft]

• Add new DeviceEndpointClient + AssetClient classes
  • These clients provide APIs for updating/getting status of the device/asset as well as APIs for forwarding sampled datasets/received events
    • Previously, this was all done via the base connector APIs, but those required the user to pass in device name, inbound endpoint name, asset name, etc. which was unnecessarily cumbersome.
• Add some helper functions to the asset status class to help users treat the lists more like dictionaries
  • The intended practice for an asset status is for the list of dataset statuses to only contain one entry per dataset (the latest status) even though the data structure allows for more than one
• Update existing sample connectors to report device + asset status
• Defer dataset/event message schema registration until they forward a sampled dataset/received event
  • This is when we will be able to infer message schemas later. Making this change now to avoid it being a breaking change later

Intentionally not included in this PR (saved for post-GA if requested):

• Relying on notification APIs in connectors to learn when an asset/device is updated rather than just polling the current status before every update
• Caching any device/asset status in base connector