Generate certs and private keys for tests

[tsaarni]

This PR refactors feature tests:

• Replace hardcoded test certs and private key with generated ones
• Replace secrets instantiated inline with functions that construct them

There are still certificates in internal/fixture/certificate_constants.go which are used outside featuretests. Since the change is getting large and risk of merge conflicts with other PRs increase, I'd prefer including only featuretest related certs in this PR.

Related to #6079 (comment)

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (4ae0bfa) 78.85% compared to head (e3a0acf) 78.82%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6100      +/-   ##
==========================================
- Coverage   78.85%   78.82%   -0.03%     
==========================================
  Files         138      138              
  Lines       19731    19765      +34     
==========================================
+ Hits        15558    15580      +22     
- Misses       3870     3878       +8     
- Partials      303      307       +4     

Files	Coverage Δ
internal/featuretests/v3/envoy.go	99.08% <100.00%> (-0.01%)	⬇️
...ernal/provisioner/objects/deployment/deployment.go	88.98% <100.00%> (-0.05%)	⬇️
internal/featuretests/kubernetes.go	83.78% <73.80%> (-16.22%)	⬇️

[skriss]

nice 👍

[izturn]

nice refactor