fix: Recompute Content-Digest header value when signing content-digest component

[dadrus]

This PR changes the signing behavior for the content-digest component.

When content-digest is configured as one of the signature components, the signer now always recomputes the Content-Digest header from the current message body and replaces any existing value.

Previously, an existing Content-Digest header was preserved. If that header was stale or incorrect, signing could produce a message that fails verification because the signed digest no longer matches the body.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.52%. Comparing base (0ae7976) to head (ea69904).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
- Coverage   94.53%   94.52%   -0.01%     
==========================================
  Files          18       18              
  Lines        1317     1315       -2     
==========================================
- Hits         1245     1243       -2     
  Misses         37       37              
  Partials       35       35              

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