Implement XML-RPC endpoint discovery

[crazytonyli]

The XML-RPC endpoint discovery process uses the results of the REST API discovery. That means we can be sure that the site is a WordPress site and use the wp-json results in the XML-RPC discovery process.

The XML-RPC endpoint URL can be found in the process below:

1. Parse the site homepage HTML to get an RSD link, which is typically <site-url>/xmlrpc.php?rsd
2. Parse the RSD link content, which is an XML, to get the XML-RPC endpoint URL.

We'll check the discovered URL and the default <site-url>/xmlrpc.php (if they are not the same). We can verify if it's disabled by sending a system.listMethods XML-PRC call.

Here are a couple of examples.

// Success
$ curl -s --data-raw '<?xml version="1.0"?><methodCall><methodName>system.listMethods</methodName></methodCall>' https://www.nasa.gov/xmlrpc.php

// Failure
$ curl -s --data-raw '<?xml version="1.0"?><methodCall><methodName>system.listMethods</methodName></methodCall>' https://www.whitehouse.gov/xmlrpc.php

[crazytonyli]

I want to add another test case for the XMLRPCDisabledReason::ByPlugin error. I haven't configured the test site yet, but will add one once the test site is ready.

[oguzkocer]

Looks good. I could offer minor suggestions for Rust, but I don't think any of them are worth holding up the PR, so I'd rather we merge it sooner rather than later.