feature:support ipv6 address (#280)

Problem Summary:
Now the etcd client address do not support host format such as
http://[ipv6]:port.

and [RFC
3986](https://datatracker.ietf.org/doc/html/rfc3986#section-3.2.2)
```
A host identified by an Internet Protocol literal address, version 6
[[RFC3513](https://datatracker.ietf.org/doc/html/rfc3513)] or later, is distinguished by enclosing the IP literal
within square brackets ("[" and "]").  This is the only place where
square bracket characters are allowed in the URI syntax.  In
anticipation of future, as-yet-undefined IP literal address formats,
an implementation may use an optional version flag to indicate such a
format explicitly rather than rely on heuristic determination.

      IP-literal = "[" ( IPv6address / IPvFuture  ) "]"
```
the ipv6 address is distinguished by enclosing the IP literal within
square brackets ("[" and "]").

What is changed and how it works:
if we find the host part of a URL starts with a "[" and ends with a "]".
it will be specifically judged whether it is a valid IPv6 address. If it
is a valid address, it can be returned directly without call the
`getaddrinfo` interface.

src/SyncClient.cpp

@@ -8,6 +8,7 @@
 #include <netinet/in.h>
 #endif
 
+#include <arpa/inet.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/types.h>
@@ -101,6 +102,7 @@ static std::string string_join(std::vector<std::string> const& srcs,
 static bool dns_resolve(std::string const& target,
                         std::vector<std::string>& endpoints, bool ipv4 = true) {
   std::vector<std::string> target_parts;
+  bool ipv6_url{false};
   {
     size_t rindex = target.rfind(':');
     if (rindex == target.npos) {
@@ -110,7 +112,17 @@ static bool dns_resolve(std::string const& target,
 #endif
       return false;
     }
-    target_parts.push_back(target.substr(0, rindex));
+
+    std::string host(target.substr(0, rindex));
+
+    // host format is [ipv6]
+    if (!ipv4 && !host.empty() && host[0] == '[' &&
+        host[host.size() - 1] == ']') {
+      host = target.substr(1, rindex - 2);
+      ipv6_url = true;
+    }
+
+    target_parts.push_back(host);
     target_parts.push_back(target.substr(rindex + 1));
   }
 
@@ -132,6 +144,16 @@ static bool dns_resolve(std::string const& target,
   }
 #endif
 
+  if (ipv6_url) {
+    // check valid ipv6
+    struct sockaddr_in6 sa6;
+    if (inet_pton(AF_INET6, target_parts[0].c_str(), &(sa6.sin6_addr)) == 1) {
+      endpoints.emplace_back(target);
+      return true;
+    }
+    return false;
+  }
+
   struct addrinfo hints = {}, *addrs;
   hints.ai_family = ipv4 ? AF_INET : AF_INET6;
   hints.ai_socktype = SOCK_STREAM;

tst/EtcdResolverTest.cpp

@@ -9,6 +9,9 @@ static const std::string etcd_v4_url =
     etcdv3::detail::resolve_etcd_endpoints("http://127.0.0.1:2379");
 static const std::string etcd_v6_url =
     etcdv3::detail::resolve_etcd_endpoints("http://::1:2379");
+// http://[ipv6]:port url
+static const std::string etcd_ipv6_url =
+    etcdv3::detail::resolve_etcd_endpoints("http://[::1]:2379");
 
 TEST_CASE("test ipv4 connection") {
   std::cout << "ipv4 endpoints: " << etcd_v4_url << std::endl;
@@ -20,4 +23,8 @@ TEST_CASE("test ipv6 connection") {
   std::cout << "ipv6 endpoints: " << etcd_v6_url << std::endl;
   etcd::Client etcd(etcd_v6_url);
   REQUIRE(etcd.head().get().is_ok());
+
+  std::cout << "ipv6 endpoints: " << etcd_ipv6_url << std::endl;
+  etcd::Client etcd1(etcd_ipv6_url);
+  REQUIRE(etcd1.head().get().is_ok());
 }