[lldb] [Process] Introduce protocol extension support API

Introduce a NativeProcessProtocol API for indicating support for
protocol extensions and enabling them.  LLGS calls
GetSupportedExtensions() method on the process factory to determine
which extensions are supported by the plugin.  If the future is both
supported by the plugin and reported as supported by the client, LLGS
enables it and reports to the client as supported by the server.

The extension is enabled on the process instance by calling
SetEnabledExtensions() method.  This is done after qSupported exchange
(if the debugger is attached to any process), as well as after launching
or attaching to a new inferior.

The patch adds 'fork' extension corresponding to 'fork-events+'
qSupported feature and 'vfork' extension for 'vfork-events+'.  Both
features rely on 'multiprocess+' being supported as well.

Differential Revision: https://reviews.llvm.org/D100153