Enable catalyst source copying

[mjh1]

• This enables source copying at catalyst for upload tasks (and not transcode-file) by setting the SourceMp4 flag on the catalyst request.
• Removed unused function removeCredentials

[victorges]

LGTM anyway, but made some comments regarding the "loosiness" of the current interface and the location where some things are being done (complement vs process).

All comments are optional, feel free to merge as is.

[codecov]

Codecov Report

Merging #204 (e461247) into main (84726bf) will increase coverage by 0.62334%.
The diff coverage is 12.50000%.

@@                 Coverage Diff                 @@
##                main        #204         +/-   ##
===================================================
+ Coverage   14.77750%   15.40084%   +0.62334%     
===================================================
  Files             19          19                 
  Lines           2382        2370         -12     
===================================================
+ Hits             352         365         +13     
+ Misses          2005        1981         -24     
+ Partials          25          24          -1     

Impacted Files	Coverage Δ
task/import.go	0.00000% <0.00000%> (ø)
task/upload.go	22.03742% <13.33333%> (+2.76764%)	⬆️

... and 1 file with indirect coverage changes

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Impacted Files	Coverage Δ
task/import.go	0.00000% <0.00000%> (ø)
task/upload.go	22.03742% <13.33333%> (+2.76764%)	⬆️

... and 1 file with indirect coverage changes