An index spec's "weights" field is normally an object, but it can also be the string "$**". In SERVER-52545 we updated listIndexes to return an error if any index spec in the catalog does not match its expected format, and we incorrectly required "weights" to always be an object. Update the IDL definition for index specs to permit an object or a string.

(Note, "weights" has some other surprising behavior which I'm not fixing here. Deferred to SERVER-54712.)