Set up command construction to support generic iterables and mappings

[LewisGaul]

First step in working on #584.

• Command.add_args_list() previously had a misleading name and type annotation - it is used for accepting either a list or a single item. Renamed to Command.add_args_iterable_or_single().
• Added Command.add_args_iterable() for the case you know you're dealing with an iterable (more efficient than always creating a new list).
• Updated utils.to_list() to support generic iterables (still always returns a list, as per the name - my plan is to phase out its use where it's not needed).
• Updated utils.format_dict_for_cli() to support generic mappings.
• Added Command.add_args_mapping() as a convenience that wraps utils.format_dict_for_cli(), which will allow tidying up code that constructs commands.

Next step is to tidy up the components' code that constructs commands, allowing arguments to be Iterable and Mapping in place of List and Dict. With these changes it should be fairly straightforward, simply calling the correct method on the Command class.

Here's an example of what upcoming changes look like: LewisGaul/python-on-whales@command-construction...command-construction-image

[gabrieldemarmiesse]

Looks good overall. We can drop the speed optimisation. I don't think anyone is going to notice.

I usually refuse code that is added "to make things faster" unless there is a benchmark showing that the speed increase is noticeable.