Refactor STS client operation modules

[GatewayJ]

Type of Change

• Refactor
• New Feature
• Bug Fix
• Documentation
• Performance Improvement
• Test/CI
• Other: N/A

Related Issues

• N/A

Summary of Changes

• Reorganize src/sts/rustfs_client.rs by splitting admin/STS/S3/pool helper logic into dedicated submodules.
• Move shared request-signing and credential/response helpers into core_ops.rs and helpers.rs.
• Move RustfsAdminClient tests to src/sts/tests.rs while preserving existing coverage behavior.

Checklist

• I have read and followed the CONTRIBUTING.md guidelines
• Passed make pre-commit (fmt-check + clippy + test + console-lint + console-fmt-check)
• Added/updated necessary tests
• Documentation updated (if needed)
• CHANGELOG.md updated under [Unreleased] (if user-visible change)
• CI/CD passed (if applicable)

Impact

• Breaking change (CRD/API compatibility)
• Requires doc/config/deployment update
• Other impact: N/A

Verification

git show --stat --oneline --no-patch

Additional Notes

• Commit created and pushed to my/feature/leader-election.

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