Replace dependency on the XML-RPC extension

Currently Moodle relies on the XML-RPC extension to implement the XML-RPC related features. As the official docs states:

This extension is EXPERIMENTAL. The behaviour of this extension including the names of its functions and any other documentation surrounding this extension may change without notice in a future release of PHP. This extension should be used at your own risk.

In MDLSITE-4726 and MDL-57775 we ran into a weird behaviour of the extension when it comes to encoding / decoding the non-latin payload and it was pretty annoying to not be able to rely on the functions documentation.

We have never been really happy about the XML-RPC status and it has been discussed in the past with the MNet developers that ideally we should replace it with a standalone library written in pure PHP.

In MDL-57775, Cameron agreed:

+1 for replacing the extension. There are even discussions on the PHP mailing list about moving it to PECL - so replacing it is probably in our best interests

Let us replace all the functions provided by the XML-RPC extension.