Feature: Added source field option for define-index-field command under CloudSearch

[akhill10]

Fixes Issue #6017

Description of changes:
Added [--source-field <value>] option for the define-index-field command under CloudSearch.

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[codecov-commenter]

Codecov Report

Merging #6045 (e3d4031) into develop (c71d1df) will decrease coverage by 0.22%.
The diff coverage is n/a.

❗ Current head e3d4031 differs from pull request most recent head ccbe2a4. Consider uploading reports for the commit ccbe2a4 to get more accurate results

@@             Coverage Diff             @@
##           develop    #6045      +/-   ##
===========================================
- Coverage    92.88%   92.66%   -0.23%     
===========================================
  Files          204      204              
  Lines        16344    16188     -156     
===========================================
- Hits         15181    15000     -181     
- Misses        1163     1188      +25     

Impacted Files	Coverage Δ
awscli/customizations/cloudsearch.py	91.30% <ø> (ø)
awscli/customizations/ecs/deploy.py	60.31% <0.00%> (-32.81%)	⬇️
awscli/customizations/configure/list.py	89.79% <0.00%> (-2.05%)	⬇️
awscli/utils.py	93.38% <0.00%> (-1.91%)	⬇️
.../customizations/datapipeline/createdefaultroles.py	84.72% <0.00%> (-1.00%)	⬇️
awscli/customizations/codeartifact/login.py	97.48% <0.00%> (-0.36%)	⬇️
awscli/customizations/configure/writer.py	99.04% <0.00%> (-0.10%)	⬇️
awscli/argprocess.py	94.28% <0.00%> (-0.07%)	⬇️
...customizations/cloudformation/artifact_exporter.py	98.65% <0.00%> (-0.07%)	⬇️
awscli/clidriver.py	95.94% <0.00%> (-0.04%)	⬇️
... and 24 more

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[justindho]

Thanks, @pawanakhil!