docs/vending-machine: Document applications protocol and message format

The "Vending Machine" blueprint takes advantage of two Linux
applications: a UI and a Control one. These applications will exchange
messages over a defined interface using a specific protocol. This
documentation provides the specification of the communication protocol,
message format, schemas and more.

Fixes https://git.ostc-eu.org/OSTC/planning/core-os/-/issues/224

Relate-to: https://git.ostc-eu.org/OSTC/OHOS/meta-ohos/-/merge_requests/282


Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Philippe Coval <philippe.coval@huawei.com>