feat: support default role assumers

[AllanZhengYP]

Issue

Resolves #2087
Resolves #1998
Resolves #2011
Resolves #1193

Replaces #2179

Description

This change provides default role assumer for the default credentials provider that calls sts:assumeRole or sts:assumeRoleWithWebIdentity under the hood. As a result, users don't need to import STS client and supply their own role assumer(like mentioned in #1193).

These assumer is exported from STS client not the packages/ folder because it can avoid circular dependency. The credential-provider-* packages having any dependency over STS client will cause the same issue. As a result this change makes the source code comply the contract that clients/ depends on packages/; lib/ depends on packages/ and clients/.

Testing

✅ It has been manually tested with credential files containing assume role chaining
✅ It has been validate with bundler(Webpack) that tree shaking work well

Additional context

This PR is based #2179 but it doen't use dynamic import, and doen't introduce separate STS clients.

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[trivikr]

LGTM, just one nit.

[aws-sdk-js-automation]

AWS CodeBuild CI Report

• CodeBuild project: sdk-staging-test
• Commit ID: 0eb0274
• Result: SUCCEEDED
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