Convert HTTP module into hex_core adapter

[cgerling]

Part of #738

[cgerling]

this was required because :hex_core headers are #{binary() => binary()} and :httpc.request/5 only accepts charlists

[ericmj]

I think it would be better to change it to binary inside Hex and only convert to/from charlists when we call the httpc functions.

[cgerling]

I thought about changing this along the hex_core migration but I think it's easier/better to do it now

[ericmj]

I think it would be better to change it to binary inside Hex and only convert to/from charlists when we call the httpc functions.

[ericmj]

We can't add dependencies other than for testing in Hex. Instead we vendor hex_core using this script https://github.com/hexpm/hex/blob/main/scripts/vendor_hex_core.sh. The hex_core module names will be prefixed with mix_.

[cgerling]

Got it, I've removed this and renamed the hex_core references

[ericmj]

Can you rebase against main so we can get CI to run? And let us know when it's ready for review again.

[ericmj]

Looks good. Can you look into why there are timeouts on older Elixir/OTP versions?

[wojtekmach]

Thanks!

[cgerling]

I'm not entirely sure why the timeout was occurring but I found out that httpc only supports binaries as header values from OTP/24 and above, so I updated the code to encode header values as charlists to, it seemed to solve the timeout as well, I tested with all versions locally to be sure.

[ericmj]

Thank you @cgerling!