karma: Add typings for 'middleware' in ConfigOptions (#26882)

Karma config accepts a list of middleware as documented here:
http://karma-runner.github.io/2.0/config/configuration-file.html

types/karma/index.d.ts

@@ -294,6 +294,18 @@ declare namespace karma {
          * @description A list of log appenders to be used. See the documentation for [log4js] for more information.
          */
         loggers?: Log4jsAppenderConfigBase[];
+        /**
+         * @default []
+         * @description List of names of additional middleware you want the
+         * Karma server to use. Middleware will be used in the order listed.
+         * You must have installed the middleware via a plugin/framework
+         * (either inline or via NPM). Additional information can be found in
+         * [plugins](http://karma-runner.github.io/2.0/config/plugins.html).
+         * The plugin must provide an express/connect middleware function
+         * (details about this can be found in the
+         * [Express](http://expressjs.com/guide/using-middleware.html) docs).
+         */
+        middleware?: string[];
         /**
          * @default {}
          * @description Redefine default mapping from file extensions to MIME-type.

types/karma/karma-tests.ts

@@ -88,6 +88,8 @@ module.exports = function(config: karma.Config) {
       'coverage'
     ],
 
+    middleware: ['foo', 'bar'],
+
     mime: {
       'text/x-typescript': ['ts', 'tsx']
     },