feat(storage) : Support custom prefix

[fjnoyp]

Duplicate of: https://github.com/aws-amplify/amplify-flutter/pull/2071 for the next branch

Modify method channels to call native layer support for custom prefix.

Issue #, if available: https://github.com/aws-amplify/amplify-flutter/issues/416

Description of changes: Call custom prefix in native side.

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[codecov-commenter]

Codecov Report

Merging #2117 (d17b238) into next (d17b238) will not change coverage. The diff coverage is n/a.

:exclamation: Current head d17b238 differs from pull request most recent head 3bddcbc. Consider uploading reports for the commit 3bddcbc to get more accurate results

@@           Coverage Diff           @@
##             next    #2117   +/-   ##
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  Coverage   42.86%   42.86%           
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  Files         114      114           
  Lines        7502     7502           
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  Hits         3216     3216           
  Misses       4286     4286           

Flag	Coverage Δ
android-unit-tests	∅ <0.00%> (∅)
flutter-unit-tests	25.08% <0.00%> (ø)
ios-unit-tests	89.06% <0.00%> (ø)

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[HuiSF]

Dart Storage S3 plugin has been merged into next, this PR is no longer needed. Thanks for the efforts!