feat: use cookie_file instead of auth

[Kodylow]

Uses the generated cookie file by sharing the bitcoin_volume instead of the user/pass, needed for new bitcoin versions > 0.26.

Fedimint doesn't look like it's picking up the cookie file though which is weird, I'm definitely setting it correctly in the container:

}bash-5.2# echo $FM_BITCOIND_COOKIE_FILE_ENV
/bitcoind_data/.cookie
bash-5.2# cat /bitcoind_data/.cookie
__cookie__:c575d3ddde3a9439a0e0d113b07280887b769bf9cdfb0954f8de0c19898c3befbash-5.2# exit

and that should be getting picked up by the bitcoincore function:

pub fn from_url_to_url_auth(url: &SafeUrl) -> anyhow::Result<(String, Auth)> {
    Ok((
        (if let Some(port) = url.port() {
            format!(
                "{}://{}:{port}",
                url.scheme(),
                url.host_str().unwrap_or("127.0.0.1")
            )
        } else {
            format!(
                "{}://{}",
                url.scheme(),
                url.host_str().unwrap_or("127.0.0.1")
            )
        }),
        match (
            !url.username().is_empty(),
            env::var(FM_BITCOIND_COOKIE_FILE_ENV),
        ) {
            (true, Ok(_)) => {
                bail!("When {FM_BITCOIND_COOKIE_FILE_ENV} is set, the url auth part must be empty.")
            }
            (true, Err(_)) => Auth::UserPass(
                url.username().to_owned(),
                url.password()
                    .ok_or_else(|| format_err!("Password missing for {}", url.username()))?
                    .to_owned(),
            ),
            (false, Ok(path)) => Auth::CookieFile(PathBuf::from(path)),
            (false, Err(_)) => Auth::None,
        },
    ))
}

but whenever I try I get jsonrpc errors when fedimint tries starting the dkg