server: proxy: rewrite modules API

server: proxy: rewrite modules API

* Add support for C++ plugins.
* Seperate between a "module" and a "plugin": a module is a shared
library that exposes a function called "proxy_module_entry_point". This
function gets a parameter to `proxyPluginsManager*`, which can be used to
register plugins.
* Refine the API of the modules infrastructure:
 * use proxyData* instead of rdpContext* when firing a hook or calling a filter.
 * use clearer names.
* Instead of having to specify a path for each module to be loaded in
the configuration, the proxy now loads modules from specificed
directory, using the CMAKE variable PROXY_PLUGINDIR.
* Add an option to specify required plugins: plugins that the proxy
wouldn't start without having them fully loaded.