Add parameter substitution

[kenji-miyake]

It is useful if we can use the parameter substitution.

<launch>
  <set_parameter name="name" value="value" />
  <let name="result" value="$(param name)" />
</launch>

SetParameter('name', 'value').execute(context)
value = Parameter('name').perform(context)

Also, since I got this error while I'm testing this, fixed it.

    @property
    def name(self) -> ParameterName:
        """Getter for name."""
>       return self.__param_dict.keys()[0]
E       TypeError: 'dict_keys' object is not subscriptable

build/launch_ros/launch_ros/actions/set_parameter.py:79: TypeError

[kenji-miyake]

@jacobperron @ivanpauno Hello, could you kindly review this, please? 🙏

[jacobperron]

Seems like a reasonable addition to me.

[jacobperron]

• Linux
• Linux-aarch64
• Windows