DNS config: validate mutually exclusive options when loading dns config.

#13074 added a config option that is mutually exclusive with the existing (resolvers, using tcp for resolution) dns config options. Currently using these options together results in a release assert being triggered.

This issue tracks adding a centralized config validation function that always runs on the main thread. This way the validation can safely throw an exception. When this validation is added the release assert in `createDnsResolver` can be deleted.