Allow-list for proxying reply headers in MTBroker filter

[Ali Ok]

Hello Knative developers,

**If you are a channel implementor, you might want to have a look!**

In https://github.com/knative/eventing/pull/6357, we're trying to implement an allow-list for proxying the headers from replies in MTBroker filter.

"For clarity / posterity, this is at the following location (**) in the data path, on the response return from the subscriber: 

[ channel ] --(subscription)-->** [ MT broker filter ] **--(trigger)--> [subscriber] "

With an earlier PR in Dec 2021, we were hoping for an easy net-win by passing through everything. However, later we realized some headers are not meant to be proxied (such as Content-Length).

With https://github.com/knative/eventing/pull/6357, I would like to introduce an allow-list (hardcoded for now) for pass-through headers.

If you are a channel implementor, please have a look at the PR. 

If your channel implementation relies on proxying all headers (https://github.com/knative/eventing/pull/5946), please shout!

Thanks

Ali