Ensured that endpoint header config is not modified in future

refs https://linear.app/tryghost/issue/ENG-674/

This will cause errors to be thrown if developers attempt to modify the shared
header config in future.

ghost/api-framework/lib/pipeline.js

@@ -188,6 +188,8 @@ const pipeline = (apiController, apiUtils, apiType) => {
     return keys.reduce((obj, method) => {
         const apiImpl = _.cloneDeep(apiController)[method];
 
+        Object.freeze(apiImpl.headers);
+
         obj[method] = async function wrapper() {
             const apiConfig = {docName, method};
             let options;