feat: Implement API to set NTP servers

[hanyux-nv]

Added a new Redfish API method set_ntp_servers that configures a list of NTP servers of the BMC.

See issue #548.

[spydaNVIDIA]

lgtm!

[krish-nvidia]

For Dells, you have to issue the following redfish call. Out of experience, setting them in NetworkProtocols doesn't work 😅

curl -sku "username:password" \
  -X PATCH \
  -H "Content-Type: application/json" \
  -d '{
    "Attributes": {
      "NTPConfigGroup.1.NTPEnable": "Enabled",
      "NTPConfigGroup.1.NTP1": "pool.ntp.org",
      "NTPConfigGroup.1.NTP2": "time.google.com"
    }
  }' \
  "https://<iDRAC_IP>/redfish/v1/Managers/iDRAC.Embedded.1/Attributes"

[krish-nvidia]

See comment above ^

[krish-nvidia]

For HPEs, you have to use /redfish/v1/Managers/1/DateTime to set the NTP servers. Other vendors might require other paths as well so I'd try them out on each machine to make sure it works.

[krish-nvidia]

AMI servers require PATCH's to be sent with an If-Match header. Instead of delegating it to the standard implementation, do the following:

        if servers.is_empty() {
            return Ok(());
        }
        let url = format!("Managers/{}/NetworkProtocol", self.s.manager_id());
        let body = HashMap::from([(
            "NTP",
            serde_json::json!({
                "NTPServers": servers,
                "ProtocolEnabled": true,
            }),
        )]);
        self.s.client.patch_with_if_match(&url, body).await

[krish-nvidia]

Unused slots keep stale values when fewer than 3 servers are passed into set_ntp_servers. We should explicitly blank them out so the set is authoritative. Something like:

let mut attrs = HashMap::from([("NTPConfigGroup.1.NTPEnable", "Enabled")]);
const NTP_KEYS: [&str; 3] = [
    "NTPConfigGroup.1.NTP1",
    "NTPConfigGroup.1.NTP2",
    "NTPConfigGroup.1.NTP3",
];
for (i, key) in NTP_KEYS.into_iter().enumerate() {
    // blank unused slots so the set is authoritative
    attrs.insert(key, servers.get(i).map_or("", String::as_str));
}

[krish-nvidia]

You can do something like this instead:
let static_ntp_servers: Vec<String> = servers.iter().take(2).cloned().collect();

Also we should log when a user passes in more than 2 servers:

if servers.len() > 2 {
    tracing::warn!("iLO supports at most 2 static NTP servers; ignoring {} extra", servers.len() - 2);
}

[krish-nvidia]

nit: extra comma next to self.manager_id()

[krish-nvidia]

Also, delta_powershelf.rs is missing set_ntp_servers(), the library won't compile otherwise.