Merge pull request grpc#7771 from y-zeng/cares_buildin

Async DNS resolver with pre-configured c-ares

.gitmodules

@@ -23,3 +23,7 @@
 [submodule "third_party/boringssl-with-bazel"]
 	path = third_party/boringssl-with-bazel
 	url = https://boringssl.googlesource.com/boringssl
+[submodule "third_party/cares/cares"]
+	path = third_party/cares/cares
+	url = https://github.com/c-ares/c-ares.git
+	branch = cares-1_12_0

BUILD

@@ -67,6 +67,7 @@ grpc_cc_library(
         "grpc_lb_policy_pick_first",
         "grpc_lb_policy_round_robin",
         "grpc_load_reporting",
+        "grpc_resolver_dns_ares",
         "grpc_resolver_dns_native",
         "grpc_resolver_sockaddr",
         "grpc_secure",
@@ -850,6 +851,27 @@ grpc_cc_library(
     ],
 )
 
+grpc_cc_library(
+    name = "grpc_resolver_dns_ares",
+    srcs = [
+        "src/core/ext/resolver/dns/c_ares/dns_resolver_ares.c",
+        "src/core/ext/resolver/dns/c_ares/grpc_ares_ev_driver_posix.c",
+        "src/core/ext/resolver/dns/c_ares/grpc_ares_wrapper.c",
+    ],
+    hdrs = [
+        "src/core/ext/resolver/dns/c_ares/grpc_ares_ev_driver.h",
+        "src/core/ext/resolver/dns/c_ares/grpc_ares_wrapper.h",
+    ],
+    language = "c",
+    deps = [
+        "grpc_base",
+        "grpc_client_channel",
+    ],
+    external_deps = [
+        "cares",
+    ],
+)
+
 grpc_cc_library(
     name = "grpc_resolver_sockaddr",
     srcs = [