rclc_parameter: Allow providing custom parameter descriptors

[bjsowa]

The parameter descriptors returned by the describe_parameters service can include additional information like description of the parameter, read_only flag or value ranges. This information is, for example, utilized by rqt_reconfigure. I think it would be a cool feature to allow providing custom descriptors for the parameters, similarly to how it is done in rclcpp.

[pablogs9]

We are making a redesigning of the parameter server in micro-ROS right now. Our idea now is to divide the functionality into two modes a complete mode and a low memory mode because we found that the current implementation is really heap hungry.

Maybe we can integrate these descriptors in the complete mode CC: @Acuadros95 ?

[Acuadros95]

No problem on adding this functionality, including read_only flag and value ranges.

Maybe we can integrate these descriptors in the complete mode

[Acuadros95]

Hi @bjsowa, we just added a lot of functionality on rclc_parameter: #274.
Check it out! It included the describe parameters extra info.