main: Add the ability to load the remote plugin

main: Add the ability to load the remote plugin

Streaming to 'remote' outputs needs the remote-plugin library.
We build it automatically if we determine that the gst dependencies are
satisfied, otherwise we're using a stub version for it.

Unfortunatelly, upstream doesn't provide necessary header for the
remoting.h so we copy-paste directly. Will follow up with upstream
to provide headers for the plugins.

Bug-AGl: SPEC-3280

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ide277b402c511ed075fa1c5aaaf7770b50359c35