Add forward declaration for jets and b-tagging

Dan Guest requested to merge dguest/athena:bjet-fwd-headers into master

The b-tagging and jet headers include a lot of other headers. This is my attempt at reducing this a bit. I added xAODJet/JetFwd.h and xAODBTagging/BTaggingFwd.h, and the corresponding headers for the containers.

I'm not sure if everything is being done correctly, I should ask @christos who I think implemented something like this for egamma. Also tagging @wbalunas and @duperrin.

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