Add Event Descriptions

[Yadunund]

This PR adds Descriptions for items listed in #43

[Yadunund]

@mxgrey this is ready for review. There were some linker errors when compiling and hence I had to implement rmf_task_sequence::detail::Backup and some inherited functions in rmf_task_sequence::Task. For the latter I have left most of it as TODOs.

[mxgrey]

This is only a partial review with some comments of things that I think could be tweaked.

Overall this PR is looking good.

[mxgrey]

I don't see why this class would be in a detail namespace. If it gets touched by a user of the library then it should be in a normal public API namespace. If it doesn't get touched by a user of the library then we should keep it inside of src/ and we don't need to pimpl it.

[Yadunund]

Moved to events namespace fb34755

[mxgrey]

We should pimpl this class if it's in the public API.

[Yadunund]

Done fb34755

[mxgrey]

I wonder if some of these fields should be optional. I suppose an empty string could imply that it's unknown, but I always prefer having the "null type" as an entirely separate object type for explicitness.

[Yadunund]

Made address and email optional fb34755

[mxgrey]

Instead of a custom Call::Model we can just use WaitFor::Description::make_model and return its result for Call::Description::make_model.

[Yadunund]

Using WaitFor::Model for Call and SMS fb34755

[mxgrey]

Instead of an action name, I wonder if we should just accept an entire JSON payload to pass along to the command handle, and allow either side of the pipeline to do whatever they want with it.

[Yadunund]

Done fb34755