[ros2param] Add verb to describe parameters

[jacobperron]

Feature request

Feature description

Every parameter has a descriptor. It would be nice if the ros2 param CLI tool had a verb for getting the description of a parameter.

The verb could be called describe or info. I think I prefer info since it's shorty and more consistent with our other CLI tools.

For example,

$ ros2 param info foo_node bar_param
Name: bar_param
Description: This is a text description of the parameter
Type: double
Constraints:
    Read only: false
    Min: 0.0
    Max: 1.0
    Step: 0.1
    Additional constraints:  A text description of constraints (if there is one).

There might be some better ideas for formatting, but I think the Min, Max, and Step constraints should only appear if there is a range constraint.
And the Additional constraints should only appear if the string is not empty.

Implementation considerations

Each node provides a service (enabled by default) for requesting one or more parameter descriptors, so the tool can make a single service call when ros2 param info is called.
Considering that the input to the service request is a list of names, it might also make sense to support querying multiple parameters at a time in the CLI tool.

For example,

$ ros2 param info foo_node bar_param baz_param another_param
Name: bar_param
Description: This is a text description of the bar_param parameter
Type: string
Constraints:
    Read only: false
    Additional constraints:  A text description of constraints (if there is one).

Name: baz_param
Description: This is a text description of the baz parameter
Type: double
Constraints:
    Read only: true
    Min: 0.0
    Max: 1.0
    Step: 0.1

Name: another_param
Description: This is a text description of the another_param parameter
Type: integer array
Constraints:
    Read only: false