dns: lowering restrictions on dns_min_refresh_rate (#31871)

Lowering API limits so we can more easily test dns refresh.
Adding a bunch of e2e DNS refresh tests.

Risk Level: low
Testing: yes
Docs Changes: inline
Release Notes:
Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ 72c15547a7dea9735235e65e2323219d59b6a9dd

envoy/extensions/common/dynamic_forward_proxy/v3/dns_cache.proto

@@ -67,9 +67,9 @@ message DnsCacheConfig {
 
   // The minimum rate that DNS resolution will occur. Per ``dns_refresh_rate``, once a host is
   // resolved, the DNS TTL will be used, with a minimum set by ``dns_min_refresh_rate``.
-  // ``dns_min_refresh_rate`` defaults to 5s and must also be >= 5s.
+  // ``dns_min_refresh_rate`` defaults to 5s and must also be >= 1s.
   google.protobuf.Duration dns_min_refresh_rate = 14
-      [(validate.rules).duration = {gte {seconds: 5}}];
+      [(validate.rules).duration = {gte {seconds: 1}}];
 
   // The TTL for hosts that are unused. Hosts that have not been used in the configured time
   // interval will be purged. If not specified defaults to 5m.