frontend: include cookies for /-/debug/headers

Created by: keegancsmith

We have the useful debug endpoint which just echos back the headers we received. For example view https://sourcegraph.com/-/debug/headers

This is very useful for debugging issues around auth proxies/etc. We explicitly excluded cookies. This dates back to the original introduction of this endpoint many years ago, and the PR has no context why. Recently a customer would of found this field useful when debugging an internal proxy they use (in particular debugging how it interacted with bitbucket webhooking into sourcegraph).

Test Plan: visit /-/debug/headers

Fixes https://github.com/sourcegraph/sourcegraph/issues/37873