Prefer regex to manual parsing in parseURL:

Although `parseURL` used a regex to pull the authority out of the URL
being parsed, it performed manual parsing of the hostname and port.

This commit rolls the parsing of the username and password, if any,
directly into the regex. The hostname can be a name, an IPv4 or an
IPv6 address.

Fixes #2751

src/ripple/basics/StringUtilities.h

@@ -70,6 +70,8 @@ struct parsedURL
     explicit parsedURL() = default;
 
     std::string scheme;
+    std::string username;
+    std::string password;
     std::string domain;
     boost::optional<std::uint16_t> port;
     std::string path;