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 85.18519% with 4 lines in your changes missing coverage. Please review.

Project coverage is 71.54%. Comparing base (f8407bc) to head (69535d6).
Report is 60 commits behind head on main.

Files with missing lines	Patch %	Lines
FFMpegCore/FFMpeg/Arguments/MultiInputArgument.cs	88.00%	3 Missing ⚠️
FFMpegCore/FFMpeg/FFMpegArguments.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #505      +/-   ##
==========================================
- Coverage   74.31%   71.54%   -2.78%     
==========================================
  Files         113      116       +3     
  Lines        2854     2959     +105     
  Branches        0      282     +282     
==========================================
- Hits         2121     2117       -4     
- Misses        733      737       +4     
- Partials        0      105     +105     

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