feat: allow setting custom HTML templates

[feeltheajf]

Definition of Ready

• [x] 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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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[bot]]

:tada: This PR is included in version 0.3.0 :tada:

The release is available on GitHub release

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