…rvices

With this, the parameters are shown in Rqt or Foxglove Studio.

The code is already merged, so renaming makes little sense.

With this change, adding doc comments to fields of structures used
with `#[derive(RosParams)]` results in those comments being used as
parameter description.

See r2r/examples/parameters_derive.rs for how to use and test this
feature.

*BREAKING CHANGE*

This commit changes r2r public API. Previously Node::params contained
HashMap<String, ParameterValue>, now it contains HashMap<String, Parameter>.

If you previously used the ParameterValue from this HashMap, now you
can get the same by using the .value field of the Parameter structure.