Kdl include syntax

[pkleineb]

These changes aim to provide an include keyword in the kdl config files that provides support for spreading configuration over multiple files as discussed in #3376.

What I did:

• added include keyword to the kdl config
• the include keyword parses the included file(s) just like the main.kdl file gets parsed
• include keyword can take either a single file path as a string as an argument like this:

  include "path/to/kdl/config/file.kdl
  

  or take multiple paths as a list of strings:

  include {
    "path1/to/file.kdl"
    "path2/to/file.kdl"
  }
  

• the include keyword may take absolute or relative paths and does expand shell variables using shellexpand

What I didn't implement(maybe yet):

• include can only be written once per file similar to all the other config options
• most other config structs implement a to_kdl method which isn't implemented for include although I feel like it would be quite difficult
• I only implemented some quick tests that might not test all cases (let me know if I should improve on that)

I hope this summarizes everything for you guys. Tell me if I can improve on something since I am still learning rust and not all too well versed in the language. I was also not too sure where to put my functions so tell me if they should be moved. Also tell me if I should squash my 3 commits no problem. Also feel free to tell me if this shouldn't be merged to main, wasn't quite sure now.