Add support for actions

[nnmm]

Actions are essentially pub/sub + clients/services, both of which are already implemented, so this should be relatively straightforward.

[Tacha-S]

I'm adding action messages generator as a start to implementing the action server.
The service message struct is ServiceName_[Request/Responce], is this different from Rust naming rule to avoid conflicts with default service struct?
I think that the service request/response messages do not conflict with other messages, because service struct is in srv module and request/response messages in srv::rmw module.
I checked implementation PR, but can't find any discussion.
Do you know any another reasons?

[nnmm]

@esteve Please see the question above.

[esteve]

@Tacha-S I hadn't received a notification for your comment, I'm terribly sorry for the slow response. Are you still interested in actions? I started the work in #295, which has a generator for action structures, but it's still missing the code for actually sending/processing actions. Would you be interested in contributing to it? Thanks.

[Tacha-S]

I don't have much free time these days either, but I haven't lost interest, and I will submit a PR if I can make it to the point where I can contribute regarding sending/processing actions.