CFP AVAD: Add public access to FeedRange from ChangeFeedProcessorContext

[philipthomas-MSFT]

Is your feature request related to a problem? Please describe.
We have a customer that needs access to this for a complex scenario where they need to persist feed ranges as "bookmarks" along with the changed document's LSN. This is later validated to confirm whether the LSN has been processed or not. Details of the holistic view is located here.

Bookmark sample:

[
	{
		"Range": {
			"min": "",
			"max": "05C1DFFFFFFFFC"
		},
		"LSN": "0"
	}
]

Describe the solution you'd like
Give the customer access to FeedRange from ChangeFeedProcessorContext

ChangeFeedProcessorContext.FeedRange

Describe alternatives you've considered
n/A

Additional context
N/A