Add custom cacert support

While `httpc` does support TLS verification of remote server certificates, it can only do so against the client system's authorized CAs. However, it is currently not possible to provide a custom CA certificate instead of relying the the system's authorized CAs. Other HTTP clients usually do provide this option, like `curl` with `--cacert`.

The only workarounds currently are either to add the custom CA to the system's authorized CAs (which can be inconvenient or impossible in some cases), or to skip TLS verification (which is insecure).

[The `Configuration` type](https://github.com/gleam-lang/httpc/blob/16d90a1b80351b3d97e9737675b9fd34f490c187/src/gleam/httpc.gleam#L125C1-L139C1) currently only includes a `verify_tls: Bool` field. I think custom CA certificates could maybe be implemented with a `custom_ca_cert: Option(String)` field.

What do you think?