Add SRV DNS record support to http connections

**Description** <!-- A short description of your feature --> SRV records are DNS records that resolve to a hostname and a port. Being able to specify http connections with SRV resolution support is useful in contexts where external services are defined as SRV records. **Use case / motivation** One particular example applies to service discovery with dynamic port mapping. In this context, service discovery maps one or more hosts to a SRV record. When resolving that SRV record a client gets one or more hosts and ports to reach the desired service. In my particular use case I am using AWS Service Discovery for ECS with dynamic port mapping. I would like to reach my ECS service transparently from Airflow without going thru a load balancer. I would like to be able to specify an http connection with a SRV record and let Airflow resolve the actual hostname and port using SRV lookup. **Related Issues** https://github.com/apache/airflow/pull/4481 I'll be happy to provide the implementation for this