fix: Support sha-1 algo for user certificate signature

[artonge]

Depending on the server's OpenSSL config, the hashing algorithm will be different. This is needed to support servers that haven't moved to a more secure hashing algorithm yet.

Most of the changes are made in order to be able to specify the hashing algorithm when loading the public key.
So we delay loading the public key until we know which algorithm to use.

[artonge]

/backport to stable30

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.15%. Comparing base (78b58b8) to head (245eb26).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
+ Coverage   31.99%   32.15%   +0.15%     
==========================================
  Files          26       26              
  Lines        1322     1325       +3     
  Branches       52       50       -2     
==========================================
+ Hits          423      426       +3     
  Misses        897      897              
  Partials        2        2              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[backportbot]

The backport to stable30 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable30
git pull origin stable30

# Create the new backport branch
git checkout -b backport/893/stable30

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 8578ec18 c14a7a86 a044e662 245eb260

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/893/stable30

Error: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.