engine: resources: Add a new http:proxy resource

This adds a new caching http proxy resource that can be autogrouped into
the core http:server resource, and which caches and serves files that it
received from a different http server. It first pulls them down when
they are initially requested, which makes it possible for us to use this
for provisioning a Linux installation without having to pre-rsync the
entire package repository.

examples/lang/http-proxy0.mcl

@@ -0,0 +1,37 @@
+http:server ":8080" {	# by default http uses :80 but using :8080 avoids needing root!
+	#address => ":8080",	# you can override the name like this
+	#timeout => 60,		# add a timeout (seconds)
+}
+
+# you can add a raw file like this...
+http:file "/file1" {
+	data => "hello, world, i'm file1 and i don't exist on disk!\n",
+}
+
+$version = "38"
+$arch = "x86_64"
+$distroarch_http_prefix = "/tmp/os/"
+
+file "${distroarch_http_prefix}" {	# root http dir
+	state => $const.res.file.state.exists,
+}
+
+# this one is backed by the (optional) rsync
+#http:file "/fedora/releases/${version}/Everything/${arch}/os/" {
+#	path => "${distroarch_http_prefix}",
+#}
+
+# wget http://127.0.0.1:8080/fedora/bar/foox.html
+# wget http://127.0.0.1:8080/fedora/releases/38/Everything/x86_64/os/Packages/c/cowsay-3.7.0-7.fc38.noarch.rpm
+# wget https://mirrors.xtom.de/fedora/releases/38/Everything/x86_64/os/Packages/c/cowsay-3.7.0-7.fc38.noarch.rpm
+
+http:proxy "/fedora/releases/${version}/Everything/${arch}/os/" {	# same as the http:file path
+
+	cache => "${distroarch_http_prefix}", # /tmp/os/
+	#force => false, # if true, overwrite or change from dir->file if needed
+	#ttl => 60, # 60 seconds ttl (longer for most things, -1 for infinite, 0 for just proxy)
+
+	# https://mirrors.xtom.de/fedora/releases/ => http://127.0.0.1:4280/fedora/
+	sub => "/fedora/",
+	head => "https://mirrors.xtom.de/fedora/",
+}