Implement Cosmos pagination

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This didn't turn out to be very hard - check out the test to see it in action.

Suggestions for additional testing scenarios would be appreciated!

Closes #24513

[AndriySvyryd]

Also add a test where responseContinuationTokenLimitInKb value matters.

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That's trickier than it looks - even setting this to 1kb (the minimum) still passes the test. I suspect that more complex query shapes (and possibly later pages??) should make it go over 1kb, at which point we can assert a failure, but that seems brittle and dependent on internal Cosmos stuff. I've debugged to manually confirm that the value gets passed into the QueryRequestOptions - do you have some other idea on what to do here?

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@AndriySvyryd thanks for the reviewing - pushed some changes, please take a look.

Note especially the commit switching to use ResponseMessageEnumerator for regular document enumeration (and fixing #34092) - can you please take a close look?

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Design decisions:

• Make continuationToken parameter required
• Make ToPageAsync experimental on Cosmos, in view of lifting it to core later (i.e. relational implementation). This would also allow us to switch to a non-EF paging abstraction (like the ones in Azure), if that's relevant.
• Remove regions to improve team morale
• Rename maxItemCount to pageSize
• Don't provide sync ToPage()
• The results on a given page will remain an IReadOnlyList, as opposed to anything streamable (async enumerable).
• For relational, we may one day want to provide a regular enumerable over the query results, which under the hood fetches chunks/pages; this would allow efficient abort of a query in the middle. This isn't require in Cosmos since results are already fetched in multiple roundtrips. If we do, we can just implement that as a separate API over the ToPageAsync() provided in this PR.

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@dotnet/efteam made the changes as per our decision decisions.