fix(credential-provider-node): read config and credentials files only once

[AllanZhengYP]

Description

This change refactors node credential provider.

• load config and credentials INI files only once, instead of loading them in INI credentials, process credentials provider individually.
• mark loadedConfig parameters as internal to reduce the exposure of API.
• other minor code refactor.

Additional context

These changes are required for new credential provider.

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[aws-sdk-js-automation]

AWS CodeBuild CI Report

• CodeBuild project: sdk-staging-test
• Commit ID: 862614d
• Result: SUCCEEDED
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[alexforsyth]

LGTM

[codecov-io]

Codecov Report

Merging #2045 (862614d) into master (f2a47e8) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2045      +/-   ##
==========================================
+ Coverage   79.30%   79.36%   +0.06%     
==========================================
  Files         368      368              
  Lines       15132    15210      +78     
  Branches     3222     3254      +32     
==========================================
+ Hits        12001    12072      +71     
- Misses       3131     3138       +7     

Impacted Files	Coverage Δ
packages/util-waiter/src/utils/validate.ts	81.81% <0.00%> (-18.19%)	⬇️
...process-arnables-plugin/update-arnables-request.ts	91.30% <0.00%> (-8.70%)	⬇️
lib/storage/src/data-chunk/readable-helper.ts	86.84% <0.00%> (-5.47%)	⬇️
packages/s3-request-presigner/src/presigner.ts	94.73% <0.00%> (-5.27%)	⬇️
...s/middleware-bucket-endpoint/src/bucketHostname.ts	98.27% <0.00%> (-1.73%)	⬇️
packages/util-dynamodb/src/convertToAttr.ts	98.94% <0.00%> (-1.06%)	⬇️
packages/util-waiter/src/poller.ts	100.00% <0.00%> (ø)
packages/util-waiter/src/waiter.ts	100.00% <0.00%> (ø)
protocol_tests/aws-ec2/endpoints.ts	81.48% <0.00%> (ø)
packages/smithy-client/src/client.ts	76.47% <0.00%> (ø)
... and 62 more

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[github-actions]

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