[Backport 7.x] Add ability to disable the http agent (#1258)

Co-authored-by: Tomas Della Vedova <delvedor@users.noreply.github.com>

test/types/kibana.test-d.ts

@@ -90,15 +90,10 @@ expectError(
     body: 42
   }).then(console.log)
 )
-/*
- * We cannot test yet the absence of a property because tsd is still shipping
- * TypeScript v3.7, as soon as it will be 3.9, we'll uncomment the following test.
 
 // @ts-expect-error
 client.async_search.get()
 
-*/
-
 // callback api is not supported
 expectError(client.cat.count({ index: 'test' }, {}, (err: any, result: any) => {}))