Record response headers on http client spans

[lforst]

There are valid use cases where you would want to capture the response headers on an HTTP client span. E.g. when you don't control the backend so you can't instrument it.

Our current implementation doesn't allow for this. We should add functionality to capture HTTP response headers.

[mydea]

We have some code for this already for replay, we may be able to hoist this up somehow!