API endpoint for keys was accidentally changed #108

The tracker has an endpoint to generate new keys.

The response for that endpoint was like:

{
  "key": "IaWDneuFNZi8IB4MPA3qW1CD0M30EZSM",
  "valid_until": 1674804892
}

But now is like this:

{
  "key": "IaWDneuFNZi8IB4MPA3qW1CD0M30EZSM",
  "valid_until": 
    { 
      "secs": 1674804892, 
      "nanos": 423855037
    }
}

The change was introduced in this commit cab093c by @da2ce7.

The struct:

pub struct AuthKey {
    pub key: String,
    pub valid_until: Option<u64>,
 }

changed to:

pub struct AuthKey {
    pub key: String,
    pub valid_until: Option<DurationSinceUnixEpoch>,
}

Since we automatically build the json objects exposed in the API, that change was exposed to API consumers.

This change affected the [backend](More info: torrust/torrust-index#78).