Update Kafka configuration types

According to https://nodejs.org/dist/latest-v8.x/docs/api/tls.html#tls_tls_createsecurecontext_options the `ssl` options `ca`, `key`, and `cert` are all optional. In my case, i need to pass `rejectUnauthorized`, but I can't do so as the backend expects a `key`. Passing an empty string works, but everything becomes redacted in my log because of that.

Signed-off-by: Gauthier Roebroeck <gauthier.roebroeck@gmail.com>

plugins/kafka-backend/config.d.ts

@@ -33,8 +33,9 @@ export interface Config {
         | {
             ca: string[];
             /** @visibility secret */
-            key: string;
+            key?: string;
             cert: string;
+            rejectUnauthorized?: boolean;
           }
         | boolean;
       /**