Add support for secured http proxy

[pderop]

This PR provides the ability to configure an http client proxy in secure mode.
You can now specify an ssl context in the proxy spec of the http client, and a secured channel will then be established between the client and the proxy. Then, on top of the tunnel, the client can also establish a second secured connection between itself and the remote server.

You only have to call the secure method on the proxy options, see the following example:

	SslContext proxySsl = SslContextBuilder.forClient()
			.trustManager(InsecureTrustManagerFactory.INSTANCE).build();

	Http11SslContextSpec clientSsl =
			Http11SslContextSpec.forClient()
					.configure(builder -> builder.trustManager(InsecureTrustManagerFactory.INSTANCE));

	HttpClient.create()
			.proxy(ops -> ops
					.type(ProxyProvider.Proxy.HTTP)
					.host("localhost")
					.port(proxy.port())
					.secure(spec -> spec.sslContext(proxySsl)))
			.secure(spec -> spec.sslContext(clientSsl))
			...

Fixes #2165

[pderop]

closed the PR, which is actually not needed.