chore: align default credential provider API style

[Xuanwo]

Summary

This PR aligns DefaultCredentialProvider / DefaultCredentialProviderBuilder API style across services using aws-v4 as the baseline.

Changes

• Fix Oracle DefaultCredentialProvider::default() semantics to match new().
• Add Oracle regression test to ensure default() and new() stay equivalent.
• Export DefaultCredentialProviderBuilder from Volcengine provide_credential module.
• Export DefaultCredentialProviderBuilder from Google and Oracle crate roots.
• Make provide_credential module private in Oracle and Tencent (keep public re-exports).

Validation

• cargo check -p reqsign-oracle -p reqsign-volcengine-tos -p reqsign-google -p reqsign-tencent-cos
• cargo test -p reqsign-oracle test_default_matches_new