feat: Hot reloading of Signer keys store

[dadrus]

Related issue(s)

closes #1231

Checklist

• I agree to follow this project's Code of Conduct.
• I have read, and I am following this repository's Contributing Guidelines.
• I have read the Security Policy.
• I have referenced an issue describing the bug/feature request.
• I have added tests that prove the correctness of my implementation.
• I have updated the documentation.

Description

This PR makes heimdall listening to changes in configured Signer key store if secrets_reload_enabled is set to true. This way you can make use of a secret management service of your choice and rotate keys/certificates used by heimdall for JWT creation purposes.

[codecov]

Codecov Report

Attention: Patch coverage is 98.55072% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.28%. Comparing base (9abf723) to head (941e859).

Files	Patch %	Lines
internal/signer/jwt_signer.go	98.43%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1232      +/-   ##
==========================================
+ Coverage   89.24%   89.28%   +0.03%     
==========================================
  Files         269      269              
  Lines        8837     8862      +25     
==========================================
+ Hits         7887     7912      +25     
  Misses        703      703              
  Partials      247      247              

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