dynamic_forward_proxy: Support SRV query lookup in DNS cache used by the custom cluster.

Title: Support SRV query lookup in DNS cache used by the custom cluster in DFP filter.

Description:

This feature request is most likely a subset of existing issue for supporting SRV query in Envoy in general. However, it doesn't seem like there's much discussion around supporting it in Dynamic Forward Proxy filter based on the previous discussion. DNS cache used by Dynamic Forward Proxy filter currently performs A record lookup and works similarly to a cluster using Logical DNS for service discovery.

[optional Relevant Links:]

Any extra documentation required to understand the issue.