Add fixture containing some prod5 irfs from zenodo

[RuneDominik]

This intends to add a fixture for future unit-testing containing some of the prod5 irfs published on zenodo. Since downloading the files each time a new github action is triggered is time-consuming we try to cache things using github's action/cache.

ATM this PR is intended to do some live-testing with github's CI. I undraft it once everything works.
Edit: Everything should be working now

[codecov]

Codecov Report

Base: 91.05% // Head: 91.08% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (2c51a34) compared to base (0c3abf6).
Patch coverage: 94.11% of modified lines in pull request are covered.

❗ Current head 2c51a34 differs from pull request most recent head 20afc01. Consider uploading reports for the commit 20afc01 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
+ Coverage   91.05%   91.08%   +0.03%     
==========================================
  Files          41       43       +2     
  Lines        1699     1716      +17     
==========================================
+ Hits         1547     1563      +16     
- Misses        152      153       +1     

Impacted Files	Coverage Δ
pyirf/conftest.py	92.85% <92.85%> (ø)
pyirf/tests/test_resources.py	100.00% <100.00%> (ø)

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[maxnoe]

These files should also be added to gitignore or stored in a place outside the repo