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.