Refactor GraphQL File Upload Implementation

[DimuthuMadushan]

Purpose

Fixes: #2838

Examples

N/A

Checklist

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

[codecov]

Codecov Report

Merging #680 (2d2fda8) into master (694613c) will increase coverage by 0.00%.
The diff coverage is 92.00%.

@@            Coverage Diff            @@
##             master     #680   +/-   ##
=========================================
  Coverage     93.35%   93.36%           
  Complexity      367      367           
=========================================
  Files            57       57           
  Lines          4094     4097    +3     
  Branches       2101     2104    +3     
=========================================
+ Hits           3822     3825    +3     
  Misses          243      243           
  Partials         29       29           

Impacted Files	Coverage Δ
ballerina/listener_utils.bal	90.49% <92.00%> (+0.11%)	⬆️
ballerina/engine.bal	97.18% <0.00%> (-1.39%)	⬇️
ballerina/response_formatter.bal	95.83% <0.00%> (+0.79%)	⬆️

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[ThisaruGuruge]

Let's wait for @shafreenAnfar's response before merging this.

[shafreenAnfar]

LGTM! Let's make sure impacted parties are informed if there is any.