Add a reverse proxy to enable grpc-web for webapps

This is needed because there's no browser api with enough control over http/2
requests, so we need a proxy that connects to the RPC backend and handle the
communication between the app and service.

The service is named "databroker-grpc-web-proxy.service" because each proxy
instance connects to only one gRPC service, more services will be added to
connect to other gRPC services.

Bug-AGL: SPEC-4599

Signed-off-by: Roger Zanoni <rzanoni@igalia.com>
Change-Id: I3b5bf6521dc0683639520626819c6a54be839e53
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29425
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Tested-by: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>