fix: headless auth username attributes must be an array

[edwardfoyle]

The headless add auth transformer set username attributes as a string instead of an array causing CFN failures.
Note this error is only encountered in headless mode, not when using the auth walkthrough

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[kaustavghosh06]

LGTM.

[codecov]

Codecov Report

Merging #5467 into master will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5467      +/-   ##
==========================================
+ Coverage   59.03%   59.14%   +0.10%     
==========================================
  Files         402      402              
  Lines       18068    18068              
  Branches     3580     3393     -187     
==========================================
+ Hits        10667    10686      +19     
+ Misses       6747     6744       -3     
+ Partials      654      638      -16     

Impacted Files	Coverage Δ
...s/awscloudformation/utils/auth-request-adaptors.ts	40.20% <ø> (+19.58%)	⬆️
packages/amplify-util-mock/src/api/api.ts	0.00% <0.00%> (ø)
packages/graphql-mapping-template/src/print.ts	34.65% <0.00%> (ø)
packages/amplify-util-mock/src/storage/storage.ts	0.00% <0.00%> (ø)
...ges/amplify-util-mock/src/CFNParser/stack/index.ts	0.00% <0.00%> (ø)
...es/amplify-util-mock/src/api/resolver-overrides.ts	0.00% <0.00%> (ø)
...es/graphql-transformer-core/src/stripDirectives.ts	35.29% <0.00%> (ø)
.../amplify-cli-core/src/state-manager/pathManager.ts	70.42% <0.00%> (ø)
.../amplify-util-mock/src/utils/lambda/loadMinimal.ts	0.00% <0.00%> (ø)
...graphql-transformer-core/src/TransformerContext.ts	24.81% <0.00%> (ø)
... and 11 more

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[github-actions]

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