Allow passing fields to initializers in C/C++

[solonovamax]

Description

It would be very nice if fields could be passed to initializers in C/C++.

currently, the constructors for the generated structs only accept an allocator.
I think that the generated constructors should instead be able to accept any of their fields, allowing for (designated) initializers to be used.

as an example, currently in order to send a message with a geographic_msgs/GeoPose, I will have to do something like this:

geographic_msgs::msg::GeoPose pose;
pose.position = /* ... */;
pose.orientation = /* ... */;

or, I personally prefer to use auto pose = new geographic_msgs::msg::GeoPose(); as even though it's slightly longer it makes the initialization more explicit.

the current approach has two aspects I dislike about it:

1. I think it doesn't look as nice
2. you cannot mark it as const, because it needs to be modified after it's constructed.

I think it would be nice if instead of that one could write:

const auto pose = geograpgic_msgs::msg::GeoPose{
    .position = /* ... */,
    .orientation = /* ... */,
};

or

const geograpgic_msgs::msg::GeoPose pose = {
    .position = /* ... */,
    .orientation = /* ...*/,
};

I am not too familiar with C++ so I'm unsure if designated initializers can be used in conjunction with constructors, as a constructor is used for the allocator.
if they cannot be used together, then the other alternative is to provide a constructor with all the default values, so that at least the following can be done:

const auto pose = new geograpgic_msgs::msg::GeoPose(
    /* ... */,
    /* ... */
);

if constructors need to be used, then there are a couple of ways you could go about it:

1. put fields in the order they appear in in the definition
2. put fields without explicit defaults first and fields with explicit defaults afterwards (both in the order they appear in in the definition)
3. generate a bunch of different constructor with every permutation of fields that have a default

Motivation

I think it would look nicer, and it would also allow declaring things as const.

Design / Implementation Considerations

No response

Additional Information

No response