NetServices: implement hostname resolution and connection for BHttpRequest

BHttpSession::Execute() moves the request into the session, and returns a future BHttpResponse
object. Currently implemented are resolving the hostname, and opening the connection.

There is some scaffolding for the actual data transfer.

Change-Id: I5a8a7a7f8680036b91cdba4beee140bbed6bfd5a

docs/user/netservices/HttpSession.dox

@@ -13,6 +13,16 @@
 
 #if __cplusplus >= 201703L
 
+
+/*!
+	\file HttpSession.h
+	\ingroup netservices
+	\brief Provides classes and tools to schedule and execute HTTP requests.
+
+	\since Haiku R1
+*/
+
+
 namespace BPrivate {
 
 namespace Network {
@@ -153,6 +163,26 @@ namespace Network {
 */
 
 
+/*!
+	\fn BHttpResult BHttpSession::Execute(BHttpRequest &&request,
+		std::unique_ptr< BDataIO > target=nullptr, BMessenger observer=BMessenger())
+	\brief Schedule and execute a \a request.
+
+	\param request The (valid) request to move from.
+	\param target An optional data buffer to write the incoming body of the request to. This can be
+		\c nullptr if you want to use the default internal storage. If you provide a buffer, it
+		must be wrapped in a \c std::unique_ptr. This means that you transfer ownership to the
+		session. After the request is finished, you can regain ownership.
+	\param observer An optional observer that will receive the progress and status messages for
+		this request.
+
+	\return The \ref BHttpResult object that corresponds to this request, and that can be used to
+		monitor the progress.
+
+	\since Haiku R1
+*/
+
+
 } // namespace Network
 
 } // namespace BPrivate