fix: filter web hook headers

[hperl]

This enforces an allow list for the request headers sent to a web hook target.

Related issue(s)

Related to https://github.com/ory-corp/cloud/issues/6181

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.35%. Comparing base (68693a4) to head (12e2844).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4048       +/-   ##
===========================================
+ Coverage        0   78.35%   +78.35%     
===========================================
  Files           0      370      +370     
  Lines           0    26125    +26125     
===========================================
+ Hits            0    20470    +20470     
- Misses          0     4094     +4094     
- Partials        0     1561     +1561     

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