[FIX] webhook-timestamp header to use current seconds since epoch value instead of event timestamp

[jfcalvo]

Description

Using the same timestamp value in the payload and in the webhook-timestamp (this one used for verification) was causing an "message too old" error when verifying the webhook message.

Replacing it to be calculated in the moment of the webhook request should solve these associated problems.

Type of change

• Fix

How Has This Been Tested

• Test suite passing.
• Tested manually with a listener verifying the webhook message.

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.57%. Comparing base (a65eb96) to head (d14253b).
Report is 26 commits behind head on feat/add-webhooks-feature-branch.

Additional details and impacted files

@@                         Coverage Diff                          @@
##           feat/add-webhooks-feature-branch    #5521      +/-   ##
====================================================================
+ Coverage                             91.52%   91.57%   +0.05%     
====================================================================
  Files                                   158      159       +1     
  Lines                                  6345     6374      +29     
====================================================================
+ Hits                                   5807     5837      +30     
+ Misses                                  538      537       -1     

Flag	Coverage Δ
argilla-server	91.57% <100.00%> (+0.05%)	⬆️

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