feat: refactor and add ip saving

- clean up code to my new (and improved) standards
- print better miette diagnostigs for the error tests
- add an IP saving feature:
    save the last IP that successfully updated the records and usa that
    when restarting the service. This allows seamles upgrades of
    `webnsupdate` without having to manually trigger a DNS update

src/password.rs

@@ -0,0 +1,60 @@
+//! Make a password for use with webnsupdate
+//!
+//! You should call this command an give it's output to the app/script that will update the DNS
+//! records
+use std::io::Write;
+use std::os::unix::fs::OpenOptionsExt;
+use std::path::Path;
+
+use base64::prelude::*;
+use miette::{Context, IntoDiagnostic, Result};
+use ring::digest::Digest;
+
+/// Create a password file
+///
+/// If `--password-file` is provided, the password is written to that file
+#[derive(Debug, clap::Args)]
+pub struct Mkpasswd {
+    /// The username
+    username: String,
+
+    /// The password
+    password: String,
+}
+
+impl Mkpasswd {
+    pub fn process(self, args: &crate::Opts) -> Result<()> {
+        mkpasswd(self, args.password_file.as_deref(), &args.salt)
+    }
+}
+
+pub fn hash_identity(username: &str, password: &str, salt: &str) -> Digest {
+    let mut data = Vec::with_capacity(username.len() + password.len() + salt.len() + 1);
+    write!(data, "{username}:{password}{salt}").unwrap();
+    ring::digest::digest(&ring::digest::SHA256, &data)
+}
+
+pub fn mkpasswd(
+    Mkpasswd { username, password }: Mkpasswd,
+    password_file: Option<&Path>,
+    salt: &str,
+) -> miette::Result<()> {
+    let hash = hash_identity(&username, &password, salt);
+    let encoded = BASE64_URL_SAFE_NO_PAD.encode(hash.as_ref());
+    let Some(path) = password_file else {
+        println!("{encoded}");
+        return Ok(());
+    };
+    let err = || format!("trying to save password hash to {}", path.display());
+    std::fs::File::options()
+        .mode(0o600)
+        .create_new(true)
+        .open(path)
+        .into_diagnostic()
+        .wrap_err_with(err)?
+        .write_all(encoded.as_bytes())
+        .into_diagnostic()
+        .wrap_err_with(err)?;
+
+    Ok(())
+}