Fix WebSub Compliance issues in Hub

[ayeshLK]

Purpose

$subject

WebSubHub subscription/unsubscription verification should be WebSubRocks Compliant.

Fixes #1479

WebSubHub should be able to infer the event-type for content-distribution based on content-type and request-body.

Fixes #1460

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #211 (f393062) into main (b5c860c) will increase coverage by 1.03%.
The diff coverage is 84.03%.

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
+ Coverage   83.67%   84.71%   +1.03%     
==========================================
  Files           9       11       +2     
  Lines         784      785       +1     
  Branches      454      460       +6     
==========================================
+ Hits          656      665       +9     
+ Misses        128      120       -8     

Impacted Files	Coverage Δ
websubhub-ballerina/utils.bal	77.92% <ø> (-3.83%)	⬇️
websubhub-ballerina/http_service.bal	85.43% <60.00%> (+4.61%)	⬆️
websubhub-ballerina/subscription.bal	80.64% <80.64%> (ø)
websubhub-ballerina/content_distribution.bal	96.22% <96.22%> (ø)

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