Automatically define basic convenience functions for messages types

[tomerarnon]

• Equality would be useful to automatically have. I haven't looked too deeply into the automatic type generation from message names, but probably using something like AutoHashEquals.jl would be easy enough.

• I'd also really appreciate having keyword constructors by default. Using Base.@kwdef in the constructor code should allow for that easily. No argument constructors like Header() are already allowed by default, but not Header(frame_id = "this")

[jdlangs]

Both sound like good ideas. I'm not actively developing for this repo right now but I'd be happy to support efforts with conversation/reviews.