Support nested form params in http-client

[CambodianCoder]

Noticed that this library doesn't appear to natively supported nested form params on requests.

For example, with this library, one could attempt to send a post request with form-params of {"foo" {"bar" true}, "baz" [{"quix" 0 }]}, but the server fails to decode the nested json correctly. However, sending form-params of the format {"foo[bar]" true, "baz[0]" {"quix" 0 } succeeds.

It looks like other clojure http libraries have implemented support for this, at
clj-commons/clj-http-lite#43, and https://github.com/clj-commons/clj-http-lite/pull/44/files#diff-eca06351807a8cbec22e826d3ee44ae631af3a29af8b7cfc6734d5824f89afa2R233.

I was wondering if babashka/http-client could also support nested form params natively?

[borkdude]

Sure, a Github issue + PR is welcome