Replace XmlRpcCpp with the one available on Debian

[drdanz]

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The xmlrpcpp library is available on debian (libxmlrpcpp-dev), we have a customized version in the extern/xmlrpcpp folder. AFAIK the changes are just for

• Building on windows
• Move the library in a namespace to avoid conflicts

This library is used only for the xmlrpc carrier.

We should:

• [ ] Investigate if the "official" library works on windows now.
• [ ] Find out if it is available on Windows and distributed with some package manager (vcpkg, conda)
• [ ] Check if we have some special bugfix, and eventually propose them upstream
• [ ] Port the xmlrpc carrier to the "official" library.