Unnecessary APIs have been removed from nm-setting-proxy, client like
nm-connection-editor are expected to create a PAC script snippet the load
the location of file in NM.

The API of NMProxyConfig exposes @proxies and @excludes as strv values.
There is no need to track those values internally as a GPtrArray and
then clone them in the getters (especially, since the entire NMProxyConfig
API is internal to core.

Thereby, fix a few memory leaks in add_proxy_config() and some
style fixes for { }.

These are final/sealed types (that is without subclasses). We can
hide the type structures and neatly embed the private data there.

A new config object NMProxyConfig has been implemented inside core
which contains proxy parameters.