Add custom headers and parameters to the unsubscription request

[ayeshLK]

Purpose

$subject

Part of: #4795

Examples

N/A

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec
• Checked native-image compatibility

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.15% 🎉

Comparison is base (769ac59) 86.76% compared to head (3231b84) 86.91%.
Report is 9 commits behind head on 2201.7.x.

Additional details and impacted files

@@              Coverage Diff               @@
##             2201.7.x     #635      +/-   ##
==============================================
+ Coverage       86.76%   86.91%   +0.15%     
  Complexity         38       38              
==============================================
  Files              15       15              
  Lines             710      711       +1     
  Branches          355      355              
==============================================
+ Hits              616      618       +2     
+ Misses             89       88       -1     
  Partials            5        5              

Files Changed	Coverage Δ
ballerina/annotation.bal	80.00% <ø> (ø)
ballerina/subscriber_client.bal	97.95% <100.00%> (+0.04%)	⬆️

... and 1 file with indirect coverage changes

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