weston-ini-conf/core.cfg: Bring in shell-ext-client

weston-ini-conf/core.cfg: Bring in shell-ext-client

Rather than messing up the ini file, bring it in from the core
configuration file. This allows to start-up the gRPC proxy server at
start-up.

This is a follow from Revert "weston-ini-conf: Add agl-shell-grpc-server"
This reverts commit 2f40613cbf42fd378feea57ccefbb3215ecc1355.

Change-Id: I40ab339fe73efb07ac1e86e6778764a553a3cabe
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/28441
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
(cherry picked from commit db9539b90921538b2ad3e61d74124b0c8d8e2e4e)
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/28477