[Feature]: Add http_response Lua hook to allow customization of HTTP responses

[Caellian]

When using HTTP backend, Conky currently hardcodes a somewhat crude HTML response.

#2394 fixes a deadlock with formatting that was reported in #1649 by introducing a mutex for the string passed to microhttpd. This mutex was coincidentally a missing feature that was needed to sync Lua to sendanswer used by microhttpd.

It's possible now to let users generate the response they want the HTTP backend producing.

It would be ideal if the hook return value was a table that also includes headers besides just the content body - that would allow e.g. JSON responses.

I'm assuming that's what @ctrlcctrlv was proposing in #1649,

[sgtanvi]

Hi! I'd like to take a crack at this if it's still up for grabs. Will read through #2394 and #1649 for context and post here once I have a rough plan.

[Caellian]

Yeah, feel free to make a PR 😊

Those issues might prove to be more noise than actually related to this one. I'd start by inspecting the HTTP backend which is relatively simple, and looking into some existing hooks in llua.h.