Remove sync I/O from TokenFileWebIdentityCredentials

[gabegorelick]

See #3802

Checklist

• npm run test passes
• changelog is added, npm run add-change

[github-actions]

Greetings! We’re closing this issue because it has been open a long time and hasn’t been updated in a while and may not be getting the attention it deserves. We encourage you to check if this is still an issue in the latest release and if you find that this is still a problem, please feel free to comment or open a new issue.

[gabegorelick]

Still an issue:

aws-sdk-js/lib/credentials/token_file_web_identity_credentials.js

Line 163 in f481a23

var oidcToken = fs.readFileSync(params.envTokenFile, {encoding: 'ascii'});