Allow Middleware to alter Functions output for ServiceBus Message Binding

### Description

Similar to the issue reported here - https://github.com/Azure/azure-functions-dotnet-worker/issues/340

I have a requirement to use Middleware in Isolated Functions to add metadata to Service Bus messages on all Functions that are publishing messages. Such as Subject/Label, Correlation ID, etc.

In Middleware you can seemingly access the Binding and see the value of the Body of the message you will send, but there is no ability to set the Message Headers.

Is it possible to expose this functionality similar to how you can alter an HTTP Response?

Start by reviewing the related issue #340 and the middleware and Service Bus message-binding behavior described here. Determine where outgoing message metadata is represented and what extension point would allow middleware to alter it; done means middleware can set Service Bus metadata such as Subject/Label and Correlation ID before publishing.

Written by the indexing model from the issue text.