Add support Oracle OCI Vault for Secrets Mangement

### Description

APISIX supports Vault, GCP and AWS. I'd like to see support for Oracle OCI Vault (https://docs.oracle.com/en-us/iaas/Content/KeyManagement/home.htm)

I don't think there is a Lua library available so perhaps the approacjh would be something like:

- Call the Secrets API: https://secrets.vaults..oci.oraclecloud.com
- Implement OCI request signing (this is the tricky part)
- Handle authentication with instance principals or API keys

OCI uses a specific signature format with RSA-SHA256.

Start by reviewing the existing Vault, GCP, and AWS secret integrations in Apache APISIX to identify their entry points and tests. Then investigate the OCI Secrets API, request-signing requirements, and instance-principal or API-key authentication; done means OCI Vault secrets are supported with coverage for the selected authentication paths.

Written by the indexing model from the issue text.