Make HTTP Service Inside the WebSubHub Listener an Isolated Object

[ayeshLK]

Purpose

$subject

Fixes #1390

Checklist

• Linked to an issue
• Updated the changelog
  Added tests N/A

[codecov]

Codecov Report

Merging #160 (524107f) into main (a6de916) will decrease coverage by 0.39%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
- Coverage   83.05%   82.65%   -0.40%     
==========================================
  Files           9        9              
  Lines         773      761      -12     
  Branches      438      443       +5     
==========================================
- Hits          642      629      -13     
- Misses        131      132       +1     

Impacted Files	Coverage Δ
websubhub-ballerina/hub_listener.bal	86.66% <75.00%> (+0.30%)	⬆️
websubhub-ballerina/utils.bal	80.91% <90.00%> (ø)
websubhub-ballerina/natives.bal	93.33% <93.33%> (+4.44%)	⬆️
websubhub-ballerina/http_service.bal	78.62% <94.11%> (-3.38%)	⬇️

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