dfp: allowing the dfp cluster to do dns lookups

[alyssawilk]

This allows bypassing the DFP for most DFP use. It does not yet support the use case use_subcluster_config at which point DFP is still required.

Risk Level: meidum
Testing: MORE NEEDED
Docs Changes: n/a
Release Notes: inline
[Optional Runtime guard:] envoy.reloadable_features.dfp_cluster_resolves_hosts

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #38237 was opened by alyssawilk.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/runtime-guard-changes: FYI only for changes made to (source/common/runtime/runtime_features.cc).

🐱

Caused by: #38237 was opened by alyssawilk.

see: more, trace.

[alyssawilk]

Coverage failure is real. I think I addressed one issue in the latest push but I want to add an e2e test of request cancelation during DFP lookup. I think the code is ready for a pass regardless if you're game