feat(directlink): support Secrets Manager CRNs in authentication_key

[anaghajoshiibm]

Summary

This PR extends support for IBM Secrets Manager CRNs in the existing authentication_key field for Direct Link Gateway via SDK upgrade.

---

Changes

• Upgraded github.com/IBM/networking-go-sdk from v0.51.14 to v0.52.0
• Added support for Secrets Manager CRNs in authentication_key
• Updated MACsec-related resources and data sources to align with SDK polymorphic changes

---

Behavior

• No Terraform schema changes
• authentication_key remains a string field
• Now supports:
  • HPCS (existing)
  • Key Protect (existing)
  • Secrets Manager (new)
• CRN validation handled by backend API/SDK

---

Backward Compatibility

• No breaking changes
• Existing configurations continue to work unchanged