Unknown variants (#818)

* Add symbol provider support for renaming union variants

* Generate errors when serializing unknown variants in JSON

* Add support for serializing Unknown variants in XML

* Support optionally generating the unknown variant

* Add support for parsing unknown JSON variants

* Add unknown union support to XML

* fixup tests & clippy

* backup changes to mk-generated

* Support the unknown variant for event streams

* fix compilation warning

* fix awsquery serializer tests

* Remove conversion & lint supression

* Remove unused custom attributes

* remove some more pointless conversions

* Fix test indentation

* CR feedback

* Update Changelogs

* Remove usages of toPascalCase

* Fix eventstream error message

* Fix braces in server codegen

* Fix doublequoted string issue