[FEATURE REQUEST] file.recurse: allow merging directories from source list

[lkubb]

Is your feature request related to a problem? Please describe.
I often make use of a pattern called TOFS, where a list of source paths depending on minion metadata is passed to (usually) a file state. This allows to have default files and override them on specific systems.

The file.recurse state already handles source lists, but - in line with other file states, which only handle single files - only selects the first existing source for recursively updating a directory. This means that, e.g., when I have a default /etc/nginx configuration and I want to add a single file for specific systems, I have to duplicate the whole directory.

Describe the solution you'd like
The file.recurse state should have an optional switch that makes it recursively merge existing source items.

Describe alternatives you've considered
Duplicating/symlinking these kinds of source directories, which is very inconvenient.

Additional context
PR incoming