Sending `string_tree` / `bytes_tree` as body

Hello!

As far as I understand, sending data over a socket as one would do with httpc is one of the cases where using an iolist (or Gleam `string_tree` / `bytes_tree`) would make sense, as the BEAM optimizes for that.

I don't think this currently possible with httpc, unless I'm missing something?

If I use gleam_json to construct a JSON payload to send as a body, the docs recommend serializing to a string tree https://hexdocs.pm/gleam_json/gleam/json.html#to_string_tree but if I wanted to send that body with httpc, I'd have to convert to string or bit_array to use `dispatch` or `dispatch_bits`.

Would a PR be accepted that adds `dispatch_string_tree` and `dispatch_bytes_tree` functions?