fix: type page event listeners correctly (#6891)

This PR fixes the fact that currently if you have:

```ts
page.on('request', request => {

})
```

Then `request` will be typed as `any`. We can fix this by defining an
interface of event name => callback argument type, and looking that up
when you call `page.on`.

Also includes a drive-by fix to ensure we convert response headers to
strings, and updates the types accordingly.

utils/doclint/check_public_api/index.js

@@ -864,6 +864,27 @@ function compareDocumentations(actual, expected) {
           expectedName: 'Array<Permission>',
         },
       ],
+      [
+        'Method HTTPRequest.respond() response.body',
+        {
+          actualName: 'string|Buffer',
+          expectedName: 'Object',
+        },
+      ],
+      [
+        'Method HTTPRequest.respond() response.contentType',
+        {
+          actualName: 'string',
+          expectedName: 'Object',
+        },
+      ],
+      [
+        'Method HTTPRequest.respond() response.status',
+        {
+          actualName: 'number',
+          expectedName: 'Object',
+        },
+      ],
     ]);
 
     const expectedForSource = expectedNamingMismatches.get(source);