[Fleet] use @kbn/config-schema in Fleet API (Part 1) (elastic#192883)

## Summary

Relates elastic#184685

Readd elastic#192447 with fixes and tests
to validate that the response schemas are correct.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

x-pack/plugins/fleet/common/types/models/agent_policy.ts

@@ -71,7 +71,7 @@ export interface FullAgentPolicyInputStream {
   id: string;
   data_stream: {
     dataset: string;
-    type: string;
+    type?: string;
   };
   [key: string]: any;
 }

x-pack/plugins/fleet/common/types/models/package_policy.ts

@@ -90,6 +90,7 @@ export interface NewPackagePolicy {
     privileges?: {
       cluster?: string[];
     };
+    [key: string]: any;
   };
   overrides?: { inputs?: { [key: string]: any } } | null;
 }