Fix for issue-1075 - Move CreateSerializer out of the base class

This is a small refactor as a precursor for api-explorer work.

This adds a virtual `CanWriteType` method to `OutputFormatter` which our xml formatters use to determine which types they can and cannot serialize.
you can avoid creating the list until there is a match. This method already returns null in one case, so this is not just a perf optimization but also a consistent result from the API.