feat: allow setting custom HTML templates

[feeltheajf]

Definition of Ready

• I am happy with the code
• Short description of the feature/issue is added in the pr description
• PR is linked to the corresponding user story
• Acceptance criteria are met
• All open todos and follow ups are defined in a new ticket and justified
• Deviations from the acceptance criteria and design are agreed with the PO and documented.
• No debug or dead code
• My code has no repetitions
• Critical parts are tested automatically
• Where possible E2E tests are implemented
• Documentation/examples are up-to-date
• All non-functional requirements are met
• Functionality of the acceptance criteria is checked manually on the dev system.

closes #88

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.

Project coverage is 47.49%. Comparing base (524a649) to head (7f1cbf5).
Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/provider/identityprovider.go	53.84%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   47.36%   47.49%   +0.13%     
==========================================
  Files          21       21              
  Lines        1974     1979       +5     
==========================================
+ Hits          935      940       +5     
  Misses        981      981              
  Partials       58       58              

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[github-actions]

🎉 This PR is included in version 0.3.0 🎉

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