DnsNameResolver seems overly strict when validating service config

DnsNameResolver only allows a pre-determined set of keys used as filters, _in the entire list_:
https://github.com/grpc/grpc-java/blob/c606519a5d75b844bfc8bd243c6c9ebe1c742f96/core/src/main/java/io/grpc/internal/DnsNameResolver.java#L543-L545

That means we can never add a new filter key. That seems like a very bad idea. I don't know if this is cross-language, but the precise behavior doesn't seem nailed down in https://github.com/grpc/proposal/blob/master/A2-service-configs-in-dns.md .

CC @dfawley, @markdroth