Volcengine TOS credential providers and presign support

[ddupg]

Background

Volcengine TOS currently supports basic AK/SK signing, which is enough for the initial OpenDAL integration. This issue tracks the remaining work needed for production-ready credential support and presigned URL support.

Scope

• Support TOS presigned URL signing.
• Support STS AssumeRole credentials.
• Support ECS instance role / metadata credentials.

References

• TOS presign: https://www.volcengine.com/docs/6349/1400318
• STS temporary credentials for TOS: https://www.volcengine.com/docs/6627/102220
• IAM role assumption docs: https://www.volcengine.com/docs/6257/86373
• ECS instance role reference: https://www.volcengine.com/docs/6257/64979