[Preview] FeedRange: Add Json serialization support

[ealsur]

Description

This PR adds FeedRange.ToJsonString and FeedRange.FromJsonString to accommodate scenarios where a user might want to persist a FeedRange in a storage and have worker instances picking them up and assigning them.

IReadOnlyList<FeedRange> feedRanges = await container.GetFeedRangesAsync();
foreach (FeedRange feedRange in feedRanges)
{
     // Store feedRange.ToJsonString()
}

// Another process in another machine

string feedRangeSerialization = Store.ReadStoredFeedRange();
FeedRange feedRange = FeedRange.FromJsonString(feedRangeSerialization);
// use FeedRange, for example
FeedIterator iterator = container.GetChangeFeedStreamIterator(feedRange);

As part of the PR, the JsonConverters involved were unified (from 3 to 1), and overall serialization simplified (there were TryParse implementations before around the FeedRange implementations).

Type of change

• New feature (non-breaking change which adds functionality)