[configgrpc] endpoint validation causes failures in dns resolution

[codeboten]

The validation that was added for dns:/// makes it impossible for end users to use dns resolution as documented here: https://github.com/grpc/grpc/blob/master/doc/naming.md#detailed-design

The documentation for NewClient suggests that dns:/// is correct, but that causes the authority to be empty, making it impossible for anyone using a custom authority to configure a custom authority

[oosariemen]

Thanks for addressing this issue. It would be preferrable to be able to use a custom authority.