http: add 'sendingHeaders' event for response header manipulation

[bjohansebas]

Okay, after so much time I can finally find time for this. Meanwhile, I’m still testing locally because building Node is quite heavy.

In summary, this serves as a replacement for on-headers, which is a package that does monkey-patching. The idea is to have an event so that middlewares like compression or express-session can use it to modify headers, and for now also, in the case of compression, to transform it into a gzip stream and send the data. However, we will still need two more events to fully remove the monkey-patching of res.write and res.end used in compression, express-session, and many other middlewares

cc: @blakeembrey

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[bjohansebas]

Okay, continuing with this, Express would have an interesting situation here. While on-headers would run first and then this new event would run afterward, it could cause issues with things like compression, since it might overwrite work that was already done internally by on-headers, potentially breaking applications.

In other words, this would affect compression and other middleware. From the Express side that might not be a huge problem, but I'm not sure about the rest of the ecosystem. That's why I'm not sure whether this should be considered a semver-major change @nodejs/http @nodejs/tsc?