Support special @latest (end of stream) filter.

[JamesBirdsall]

Added support for @ latest filter in client.
In EPH, added a couple of prefab initial offset providers for user convenience, including one for latest.

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[msftclas]

@JamesBirdsall,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot