The OCID of the secret in a vault.
If the type of the credentials is {@code BASIC}, the secret must contain the username and
password in JSON format, which is in the form of {@code { "username": "", "password": "example-password" }}.
If the type of the credentials is {@code API_KEY}, the secret must contain the accessKeyId and
secretAccessKey in JSON format,
which is in the form of {@code { "accessKeyId": "", "secretAccessKey": "" }}.
Credentials for an asset source.