Add Multiple Input files

[AddyMills]

I found myself needing to add files programatically sometimes (where I don't know the amount of inputs beforehand), especially if I were to use a custom amix argument. I wasn't able to figure out how to do this with the current code.

In this PR, I added a new Argument file called MultiInputArgument.cs which takes in an IEnumerable with file paths, checks that all file paths exist, and then add the inputs to the FFMpeg string. If at least one path doesn't exist, it throws an error message that shows the files that don't exist/can't be accessed.

I also added a test for MultiInputArguments that passes.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 25.92593% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 70.51%. Comparing base (f8407bc) to head (31b117d).
Report is 7 commits behind head on main.

Files	Patch %	Lines
FFMpegCore/FFMpeg/Arguments/MultiInputArgument.cs	24.00%	19 Missing ⚠️
FFMpegCore/FFMpeg/FFMpegArguments.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #505      +/-   ##
==========================================
- Coverage   74.31%   70.51%   -3.81%     
==========================================
  Files         113      116       +3     
  Lines        2854     2937      +83     
  Branches        0      284     +284     
==========================================
- Hits         2121     2071      -50     
- Misses        733      758      +25     
- Partials        0      108     +108     

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