grpc-proxy: Init gRPC

grpc-proxy: Init gRPC

This brings in support for accessing agl-shell protocol indirectly by
using a gRPC interface which bridges the communication between a
particular client (the client issuing gRPC requests) and the AGL
compositor which does that by re-using the same agl-shell protocol.

In order to achieve that, and further more, to avoid having ifdefs code
in the compositor and deal with threading, we instead resorted to using
a helper client.

On one side this helper implements the gRPC server API,
and on the other, a wayland native client that implements the
agl_shell interface.

It uses the agl_shell_ext interface added
previously to communicate with the compositor that it requires access
to agl_shell interface as well. The helper expects that agl_shell interface
was already bounded to another client before starting it so it waits
until that happens and then it implements the protocol specification,
for each interface.

Launching the helper client automatically can be done by adding the
following entry to the ini file:

[shell-client-ext]
command=/path/to/agl-shell-grpc-server

The gRPC server implementation only handles the agl_shell interface
until to this point, specifically, the activate_app request, and the
events that were adedd with version 3 of the agl-shell protocol.

Also the implementation uses the Reactor pattern, with Callback service
that greatly simplifies the async version and avoids putting locks to
to handle multiple clients. This should allow multiple clients being
connected to the gRPC server and receive events / send requests.

Bug-AGL: SPEC-4503
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ie870da3caa138394d8dd30f9d22a5552d585d63a