data/code-gen: add support for indications

data/code-gen: add support for indications

Indication (or also notifactions) are send by the modem towards the hosts.
They are similar to response, but without a request.
Some indication will send towards the host as soon the host has created a
session with a service. For other indication the host need to subscribe first.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>