Changes to enable XML Output Formatters.

Functional tests for Output Formatters are not a part of this PR. Will follow up with Functional Tests after @harshgMSFT's conneg check-in.
just fyi...In future if we plan to cache these serializers per type, then we would be caching it in two places..input & output formatters...but yeah it depends on how common it is where users use same types for incoming request and outgoing response...

/cc: @yishaigalatzer