_get ({ key, keyOptions, specOptions }) {
    throw new Error('_get not implemented')
  }
Get a secret property value from Key Value backend.
@param {string} key - Secret key in the backend.
@param {string} keyOptions - Options for this specific key, eg version etc.
@param {string} specOptions - Options for this external secret, eg role
@returns {Promise} Promise object representing secret property values.