grpc-proxy: Added set_app_output request

grpc-proxy: Added set_app_output request

This is identical to the remote role, but I feel this conveys more
information than remote role, as remote denotes that the output is
displayed on another device, which it isn't always the case (the
system has multiple outputs all connected directly).

This introduces two new additions to the agl-shell protocol, a request
to use a different output to display/show the application and an event
to inform the shell client to use as a map between the application id
and its output. The event is necessary to let the shell client know
which output to activate the application on.

This requests implements a wrapper for gRPC that maps 1-to-1 to the
agl-shell request. There's no gRPC subscription similar to the event
though.

Bug-AGL: SPEC-4673
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I070e9fdbafd5616f3a98415193bf846aeaee9a4a