Add option to split joined Quadlet options

[cgzones]

In my opinion when using more than a handful environment variables splitting them into individual Environment= directives is more readable/maintainable.
Please consider adding such an option or make it the default.

[k9withabone]

I've been thinking about how I would like to implement this, and I've settled on adding a podlet --split-option option which takes a list of Quadlet options that are normally joined together by Podlet (such as Environment) and split them up instead. Internally, an inverse of the list (i.e., the set of all joinable Quadlet options minus the given list) will be passed to the serializer (podlet::serde::quadlet::Serializer) as which Quadlet options to join together.