[ECS] [request]: Valid SRV qname for service discovery

**Tell us about your request**
Add option to use the RFC 2782-compliant `_service._proto.namespace` SRV record qname format for ECS / Cloud Map. Currently the SRV qname format is identical to the A record format: `service.namespace`.

**Which service(s) is this request for?**
ECS, Cloud Map (aka Service Discovery)

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I'm trying to use HAProxy with ECS tasks as backends, but HAProxy will only try SRV resolution if the qname [starts with an underscore](https://github.com/haproxy/haproxy/blob/v2.0.0/src/server.c#L3147-L3149). Therefore, the SRV records produced by ECS / Cloud Map cannot be used with HAProxy (or other services that rely on the spec format).

**Are you currently working around this issue?**
The current work-around is to rely solely on A records, and to be very vigilant with DNS caching, re-lookups, etc.

Start by reading RFC 2782 and the linked HAProxy source to understand the required SRV qname format and lookup behavior. Compare that requirement with the current ECS/Cloud Map behavior described here; done means an available option produces `_service._proto.namespace` SRV names usable by HAProxy.

Written by the indexing model from the issue text.