MessagePack codec

[n-sidorov]

This PR adds ability to use MessagePack serialization with either MessagePack-specific marked contracts or using DataContract attributes (requires explicit opt-in).

NOTE: quick benchmarking showed that this performs worse than both default Orleans serializer and default MessagePack serializer (since there is more "boilerplate" code required to integrate Orleans <-> MessagePack) so I've put a note in the comments.

Microsoft Reviewers: Open in CodeFlow

[ReubenBond]

@n-sidorov when you get a chance, please agree to the CLA above. EDIT: the bot seems to think you've met the CLA requirements, so perhaps this step is not necessary since you're an MSFT employee...

[scalalang2]

👀