fix(client-sts): disable auth for public assumeRole commands

[trivikr]

Issue #, if available:
Fixes: #1602

Description of changes:
Disables auth requirement for STS operations assumeRoleWithSAML and assumeRoleWithWebIdentity

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[codecov-io]

Codecov Report

Merging #1706 (0d00e73) into master (de75f7e) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1706      +/-   ##
==========================================
- Coverage   79.77%   79.76%   -0.02%     
==========================================
  Files         325      329       +4     
  Lines       12087    12575     +488     
  Branches     2553     2672     +119     
==========================================
+ Hits         9643    10030     +387     
- Misses       2444     2545     +101     

Impacted Files	Coverage Δ
...tocol_tests/aws-restxml/commands/XmlMapsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ocol_tests/aws-restxml/commands/XmlBlobsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ocol_tests/aws-restxml/commands/XmlEnumsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ocol_tests/aws-restxml/commands/XmlListsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...col_tests/aws-restjson/commands/JsonMapsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ol_tests/aws-restjson/commands/JsonBlobsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ol_tests/aws-restjson/commands/JsonEnumsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ol_tests/aws-restjson/commands/JsonListsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...tests/aws-restxml/commands/XmlAttributesCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...tests/aws-restxml/commands/XmlNamespacesCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
... and 110 more

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