c-ares resolver: add option to use nameservers as fallback (#19010)

Commit Message: c-ares resolver: add option to use name servers as fallback.
Risk Level: low - opt in behavior
Testing: existing and additional tests
Docs Changes: added
Release Notes: added

Signed-off-by: Jose Nino <jnino@lyft.com>

Mirrored from https://github.com/envoyproxy/envoy @ b1219ef0decc22040d45fe8bf2fa86bd16ea4e3c

envoy/extensions/network/dns_resolver/cares/v3/cares_dns_resolver.proto

@@ -19,11 +19,20 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 
 // Configuration for c-ares DNS resolver.
 message CaresDnsResolverConfig {
-  // A list of dns resolver addresses. If specified, the DNS client library will perform resolution
-  // via the underlying DNS resolvers. Otherwise, the default system resolvers
-  // (e.g., /etc/resolv.conf) will be used.
+  // A list of dns resolver addresses.
+  // :ref:`use_resolvers_as_fallback<envoy_v3_api_field_extensions.network.dns_resolver.cares.v3.CaresDnsResolverConfig.use_resolvers_as_fallback>`
+  // below dictates if the DNS client should override system defaults or only use the provided
+  // resolvers if the system defaults are not available, i.e., as a fallback.
   repeated config.core.v3.Address resolvers = 1 [(validate.rules).repeated = {min_items: 1}];
 
+  // If true use the resolvers listed in the
+  // :ref:`resolvers<envoy_v3_api_field_extensions.network.dns_resolver.cares.v3.CaresDnsResolverConfig.resolvers>`
+  // field only if c-ares is unable to obtain a
+  // nameserver from the system (e.g., /etc/resolv.conf).
+  // Otherwise, the resolvers listed in the resolvers list will override the default system
+  // resolvers. Defaults to false.
+  bool use_resolvers_as_fallback = 3;
+
   // Configuration of DNS resolver option flags which control the behavior of the DNS resolver.
   config.core.v3.DnsResolverOptions dns_resolver_options = 2;
 }