Forward h2c upgrade request downstream so that it gets a response [Fixes #5005]

[yawkat]

From NettyServerUpgradeHandler: "The upgrade was successful, remove the message from the output list so that it's not propagated to the next handler. This request will be propagated as a user event instead."

So, the first request is discarded. This patch sends it downstream for normal handling.

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Build is failing locally but that seems unrelated, so we'll see what CI says.

I'm not 100% on whether this patch could duplicate the request, but since it is a FullHttpRequest that is being forwarded, it should be safe – no partial data to worry about or anything. The fact that the upgradeRequest will still contain headers related to the upgrade may also be a concern, since it allows an attacker to send these headers further downstream than before. But I think that after the upgrade is complete, an attacker could send the same headers anyway.

[graemerocher]

Can you add to your comments which issue this fixes with. Example Fixes #...

[graemerocher]

Oh I see it is in the first line of the commit which was a bit too long so wrapped around

[yawkat]

Converting this to draft for now. Turns out the client will also require changes. The spec says:

After the empty line that terminates the 101 response, the server can begin sending HTTP/2 frames. These frames MUST include a response to the request that initiated the upgrade.

This is what my patch does. However, the micronaut h2c client does not expect a response for the first request that triggers the upgrade. Changing this has compatibility implications that I need to explore further.

[yawkat]

@graemerocher This PR is ready for now. It only fixes the h2c server and normal client. The streaming client does not work yet, it never gets a response, I will work on that separately. But that was broken before this change anyway.

If possible, this PR should go into 3.1.0.