feat: allow making cognito identity request without credentials

[AllanZhengYP]

Issue #, if available:
#354
#185

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[codecov-io]

Codecov Report

Merging #853 into smithy-codegen will increase coverage by 0.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           smithy-codegen     #853      +/-   ##
==================================================
+ Coverage           92.36%   92.37%   +0.01%     
==================================================
  Files                 147      148       +1     
  Lines                2907     2911       +4     
  Branches              508      508              
==================================================
+ Hits                 2685     2689       +4     
  Misses                222      222

Impacted Files	Coverage Δ
...eware-sdk-cognito-identity/src/removeAuthPlugin.ts	100% <100%> (ø)

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[aws-sdk-js-automation]

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