serve http command to serve a remote over HTTP

This implements a basic webserver to serve an rclone remote over HTTP.

It also sets up the framework for adding more types of server later.

cmd/serve/http/testdata/golden/index.html

@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+<meta charset="utf-8">
+<title>Directory listing of /</title>
+</head>
+<body>
+<h1>Directory listing of /</h1>
+<a href="one%25.txt">one%.txt</a><br />
+<a href="three/">three/</a><br />
+<a href="two.txt">two.txt</a><br />
+</body>
+</html>