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.
@kichalla we'd fix that by just caching it in one place :wink:.

This is on our list of things that need dedicated perf tests. We'll measure at some point and see if some caching is needing here.