fix(sts): fix sts regional endpoint injection under several cases

[windmgc]

Summary

This PR fixes the STS regional injection logic under the following cases:

• When AWS_STS_REGIONAL_ENDPOINTS is regional, the service.signingRegion must be the same as the current environment's region, instead of the globalSSL's us-east-1 region defined in the global region rule. This ensures that the API request sent to the regional STS endpoint has the correct signature.
• When AWS_STS_REGIONAL_ENDPOINTS is regional and user defines the same regional STS domain(for example, sts.us-east-2.amazonaws.com), ensures that the region is not being injected again inside the regional STS domain.

This PR also adds unit tests to ensure the region is defined correctly in the signed request and the endpoint is also expected under several combinations of region, AWS_STS_REGIONAL_ENDPOINTS and user-defined endpoint config.

KAG-5021

[github-actions]

Luacheck Report

2 tests   2 ✅  0s ⏱️
1 suites  0 💤
1 files    0 ❌

Results for commit 8286243.

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