fix(gatsby-plugin-google-analytics): add cookieFlags to options schema (#27923)

Author: mxstbr

packages/gatsby-plugin-google-analytics/src/gatsby-node.js

@@ -35,6 +35,7 @@ exports.pluginOptionsSchema = ({ Joi }) =>
     sampleRate: Joi.number(),
     siteSpeedSampleRate: Joi.number(),
     cookieDomain: Joi.string(),
+    cookieFlags: Joi.string(),
     name: Joi.string(),
     clientId: Joi.string(),
     alwaysSendReferrer: Joi.boolean(),