We previously removed support for `fields` in the request body, to ensure there
was only one way to specify the parameter. We've now decided to undo the
change, since it was disruptive and the request body is actually the best place to
pass variable-length data like `fields`.

This PR restores support for `fields` in the request body. It throws an error
if the parameter is specified both in the URL and the body.

Closes #86875