StreamInterceptor type doesn't support Promise intercept

The following won't compile but should/does work:

```ts
import {
StreamInterceptor,
} from 'grpc-web';

const streamAuthInterceptor: StreamInterceptor = {
async intercept(request, invoker) {
await authRequest(request);
return invoker(request);
},
};
```

removing the `StreamInterceptor` type from the variable lets it compile and it behaves as expected (adding the appropriate metadata to the request)