There is not a single example of how to handle subscriptions with custom cloud event attributes

**What content needs to be created or modified?**
It is very common to have the infrastructure handle authentication and tenant identification. This type of information normally flows in the context. In my case, an external company forwards the tenant information as custom cloud event attributes, and I cannot see a single example of how to do a declarative subscription capable of forwarding this custom attributes (in my case in .net).
Ideally I would like dapr to pass in the http headers this information but I cannot see any examples.

**Describe the solution you'd like**
A concise example of how to achieve it

**Where should the new material be placed?**
For example [here](https://docs.dapr.io/developing-applications/building-blocks/pubsub/howto-route-messages/)