stages/prompt: add basic file field

[BeryJu]

Add prompt field file type
#2631

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[codecov]

Codecov Report

Merging #3156 (91f1dd6) into main (3a00a5a) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3156      +/-   ##
==========================================
- Coverage   92.11%   91.99%   -0.12%     
==========================================
  Files         466      466              
  Lines       21110    21133      +23     
==========================================
- Hits        19444    19439       -5     
- Misses       1666     1694      +28     

Impacted Files	Coverage Δ
authentik/stages/prompt/models.py	100.00% <100.00%> (ø)
authentik/stages/prompt/tests.py	100.00% <100.00%> (ø)
authentik/admin/tasks.py	90.91% <0.00%> (-9.09%)	⬇️
authentik/events/models.py	76.30% <0.00%> (-7.03%)	⬇️
authentik/events/tasks.py	79.35% <0.00%> (-3.26%)	⬇️
authentik/stages/email/tasks.py	86.45% <0.00%> (-1.69%)	⬇️

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