Derive macro for StructuredParameters

[BCSol]

Proposal for structured parameters derive macro (#496)

Changes:

• added trait StructuredParameters
• added derive macro in new rclrs_proc_macros
• added poc some tests
• install test_msgs and example_messages with apt, without the cargo build without colon would fail to link rosidl message (I'd be happy about some help there)

TODO:

• proper documentation of traits
• default values should be made possible
• more tests
• add generic declare macro to NodeState/Node

@mxgrey before I continue here, I'd be happy about some feedback, whether this is what you had in mind or how we could improve upon this POC.

[esteve]

@balthasarschuess @BCSol thanks for the PR, it doesn't seem necessary to have a separate crate only for the macros, can you move that into rclrs? Thanks.

[balthasarschuess]

@balthasarschuess @BCSol thanks for the PR, it doesn't seem necessary to have a separate crate only for the macros, can you move that into rclrs? Thanks.

Hey @esteve :)
thanks for having a look. I think this is not possible as proc-macros require the crate to be marked as proc macro and cannot be used inside the crate they are defined in (https://doc.rust-lang.org/reference/procedural-macros.html#r-macro.proc.def)
I think it's better to have the proc macro tests inside rclrs.

I might be mistaken, but I could not find any resource claiming that this restriction has been lifted.
In case can you point me towards the resource.