Implements server side field validation behind the
`ServerSideFieldValidation` feature gate. With the
feature enabled, any create/update/patch request
with the `fieldValidation` query param set to
"Strict" will error if the object in the request
body have unknown fields. A value of "Warn"
(also the default when the feautre is enabled)
will succeed the request with a warning.
When the feature is disabled (or the query param
has a value of "Ignore"), the request will succeed
as it previously had with no indications of any
unknown or duplicate fields.
