Support `AuthClient` for `auth`

Currently the standard `auth` parameter uses an explicit list of clients that extend [`AuthClient`](https://github.com/googleapis/google-auth-library-nodejs/blob/db27f1bc31efaa9df28da2e0a1229ee3ebea0751/src/auth/authclient.ts):

```
auth?:
| string
| OAuth2Client
| JWT
| Compute
| UserRefreshClient
| BaseExternalAccountClient
| GoogleAuth;
```

Src: https://github.com/googleapis/google-api-nodejs-client/blob/main/src/apis/abusiveexperiencereport/v1.ts#L48-L55

In order to support new clients, such as [`DownscopedClient`](https://github.com/googleapis/google-auth-library-nodejs/blob/db27f1bc31efaa9df28da2e0a1229ee3ebea0751/src/auth/downscopedclient.ts#L107), `auth` can be simplified to:
```ts
auth?: string | AuthClient | GoogleAuth;
```

Related:
- https://github.com/googleapis/google-cloud-node-core/issues/779
- https://github.com/googleapis/google-api-nodejs-client/pull/2210