Replace serde_urlencoded with serde_html_form

[jplatte]

As suggested in #2906 (comment), I think it would make sense to replace serde_urlencoded with serde_html_form.

Up-front disclaimer: I maintain this crate (happy to let you co-maintain it though)

serde_qs was also brought up in the other thread, but I think it is not a good replacement because it is a separate codebase from the ground up, so it seems likely that its behavior differs from serde_urlencoded in some edge cases (though I would expect most of them to be in deserialization, which reqwest doesn't exercise AFAIK).

Let me know what you think :)
FYI, I'm in the process of moving its source code to Codeberg and making a new release that has separate Cargo features for serialization and deserialization.