Add a getter for a bearer token from a request (yhirose#1755)

* Add a getter for a bearer token from a request

* Replace a method for bearer token getter with a free function

(cherry picked from commit 4498019)

Author: bugdea1er

httplib.h

@@ -746,6 +746,8 @@ void default_socket_options(socket_t sock);
 
 const char *status_message(int status);
 
+std::string get_bearer_token_auth(const Request &req);
+
 namespace detail {
 
 class MatcherBase {
@@ -1944,6 +1946,15 @@ inline const char *status_message(int status) {
   }
 }
 
+inline std::string get_bearer_token_auth(const Request &req) {
+  if (req.has_header("Authorization")) {
+    static std::string BearerHeaderPrefix = "Bearer ";
+    return req.get_header_value("Authorization")
+        .substr(BearerHeaderPrefix.length());
+  }
+  return "";
+}
+
 template <class Rep, class Period>
 inline Server &
 Server::set_read_timeout(const std::chrono::duration<Rep, Period> &duration) {