Request retry config per service from DNS

[sebastian-alfers]

Based on grpc-proposal A6 ("gRPC Retry Design") where more details can be found.

The basic idea is that a grapc-client is constructed with additional params regarding a potential retry behavior received through a specific dns query which serves the json-encoded config. This config was previously provided by the server.

[johanandren]

Note that this is not about retrying in general but specific retry config provided for a specific service through JSON in DNS text entries.

I'm not entirely sure this is something we should act on, from what I understand the per-service retry config in DNS is not yet implemented in the regular Java gRPC client (see https://github.com/grpc/proposal/blob/master/A2-service-configs-in-dns.md, Java "in progress") and once it is, we may get it for free by bumping the Java gRPC dependency.