Event Serialisation Fix for 8.1

[gsteel]

Adds magic un/serialize methods to the webhook event to avoid deprecation errors in 8.1

[codecov]

Codecov Report

Merging #71 (db2be55) into main (aadff44) will decrease coverage by 1.45%.
The diff coverage is 90.90%.

@@             Coverage Diff              @@
##               main      #71      +/-   ##
============================================
- Coverage     84.46%   83.01%   -1.46%     
- Complexity      196      199       +3     
============================================
  Files            37       37              
  Lines           515      524       +9     
============================================
  Hits            435      435              
- Misses           80       89       +9     

Impacted Files	Coverage Δ
src/Event/WebhookEvent.php	65.38% <90.90%> (-34.62%)	⬇️

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