fix: parse public key as fallback for certificate for bearer authentication

[evanebb]

What type of PR is this?
bug

Which issue does this PR fix:
#3173

Testing done on this change:
Added automated test cases to check whether supplying RSA/ED/EC public keys instead of certificates works as expected.
Additionally, I spun up a local Zot instance and handed it an RSA public key (not a certificate), and verified that bearer authentication works as expected.

Automation added to e2e:

Will this break upgrades or downgrades?
Nope.

Does this PR introduce any user-facing change?:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rchincha]

@evanebb pls take a look at the ci failures. Thx for getting this PR started.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.60%. Comparing base (167f7e3) to head (bca1e70).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/api/authn.go	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3180      +/-   ##
==========================================
+ Coverage   90.57%   90.60%   +0.02%     
==========================================
  Files         182      182              
  Lines       32827    32836       +9     
==========================================
+ Hits        29734    29752      +18     
+ Misses       2325     2320       -5     
+ Partials      768      764       -4     

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[evanebb]

@rchincha the CI failures (at least the testing workflow) just seemed to be flaky tests, given that two runs failed at a different point, and the latest run went fine without me changing anything in those places:

• Run 1: https://github.com/project-zot/zot/actions/runs/15402395526/job/43337857746#step:7:5099
• Run 2: https://github.com/project-zot/zot/actions/runs/15402395526/job/43346685833#step:7:1044
• Run 3 (different commit, but successful run): https://github.com/project-zot/zot/actions/runs/15411930157/job/43365689742

The only blocker is Codecov complaining about 0.10% of missing patch coverage, I'll try to bump it up that last bit but you're of course free to bypass it if you want to :)

[rchincha]

lgtm