feat: [torrust#937] add version to configration file following semver.

I will fail if the provided version is not supported.

```toml
version = "2"
```

It only supports the exect match '2'.

Author: josecelano

packages/configuration/src/lib.rs

@@ -12,7 +12,7 @@ use std::sync::Arc;
 use std::time::Duration;
 
 use camino::Utf8PathBuf;
-use derive_more::Constructor;
+use derive_more::{Constructor, Display};
 use serde::{Deserialize, Serialize};
 use serde_with::serde_as;
 use thiserror::Error;
@@ -45,6 +45,63 @@ pub type Threshold = v2::logging::Threshold;
 
 pub type AccessTokens = HashMap<String, String>;
 
+pub const LATEST_VERSION: &str = "2";
+
+/// Info about the configuration specification.
+#[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Display)]
+pub struct Metadata {
+    #[serde(default = "Metadata::default_version")]
+    #[serde(flatten)]
+    version: Version,
+}
+
+impl Default for Metadata {
+    fn default() -> Self {
+        Self {
+            version: Self::default_version(),
+        }
+    }
+}
+
+impl Metadata {
+    fn default_version() -> Version {
+        Version::latest()
+    }
+}
+
+/// Info about the configuration specification.
+#[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Display)]
+pub struct Version {
+    #[serde(default = "Version::default_semver")]
+    version: String,
+}
+
+impl Default for Version {
+    fn default() -> Self {
+        Self {
+            version: Self::default_semver(),
+        }
+    }
+}
+
+impl Version {
+    fn new(semver: &str) -> Self {
+        Self {
+            version: semver.to_owned(),
+        }
+    }
+
+    fn latest() -> Self {
+        Self {
+            version: LATEST_VERSION.to_string(),
+        }
+    }
+
+    fn default_semver() -> String {
+        LATEST_VERSION.to_string()
+    }
+}
+
 #[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Clone, Constructor)]
 pub struct TrackerPolicy {
     // Cleanup job configuration
@@ -208,6 +265,9 @@ pub enum Error {
 
     #[error("The error for errors that can never happen.")]
     Infallible,
+
+    #[error("Unsupported configuration version: {version}")]
+    UnsupportedVersion { version: Metadata },
 }
 
 impl From<figment::Error> for Error {

packages/configuration/src/v2/mod.rs

@@ -251,16 +251,24 @@ use self::health_check_api::HealthCheckApi;
 use self::http_tracker::HttpTracker;
 use self::tracker_api::HttpApi;
 use self::udp_tracker::UdpTracker;
-use crate::{Error, Info};
+use crate::{Error, Info, Metadata, Version};
+
+/// This configuration version
+const VERSION_2: &str = "2";
 
 /// Prefix for env vars that overwrite configuration options.
 const CONFIG_OVERRIDE_PREFIX: &str = "TORRUST_TRACKER_CONFIG_OVERRIDE_";
+
 /// Path separator in env var names for nested values in configuration.
 const CONFIG_OVERRIDE_SEPARATOR: &str = "__";
 
 /// Core configuration for the tracker.
 #[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Default)]
 pub struct Configuration {
+    /// Configuration metadata.
+    #[serde(flatten)]
+    pub metadata: Metadata,
+
     /// Logging configuration
     pub logging: Logging,
 
@@ -326,6 +334,12 @@ impl Configuration {
 
         let config: Configuration = figment.extract()?;
 
+        if config.metadata.version != Version::new(VERSION_2) {
+            return Err(Error::UnsupportedVersion {
+                version: config.metadata,
+            });
+        }
+
         Ok(config)
     }
 
@@ -378,7 +392,9 @@ mod tests {
 
     #[cfg(test)]
     fn default_config_toml() -> String {
-        let config = r#"[logging]
+        let config = r#"version = "2"
+
+                                [logging]
                                 threshold = "info"
 
                                 [core]

share/default/config/tracker.container.mysql.toml

@@ -1,3 +1,5 @@
+version = "2"
+
 [core.database]
 driver = "MySQL"
 path = "mysql://db_user:db_user_secret_password@mysql:3306/torrust_tracker"

share/default/config/tracker.container.sqlite3.toml

@@ -1,3 +1,5 @@
+version = "2"
+
 [core.database]
 path = "/var/lib/torrust/tracker/database/sqlite3.db"
 

share/default/config/tracker.development.sqlite3.toml

@@ -1,3 +1,5 @@
+version = "2"
+
 [[udp_trackers]]
 bind_address = "0.0.0.0:6969"
 

share/default/config/tracker.e2e.container.sqlite3.toml

@@ -1,3 +1,5 @@
+version = "2"
+
 [core.database]
 path = "/var/lib/torrust/tracker/database/sqlite3.db"
 

share/default/config/tracker.udp.benchmarking.toml

@@ -1,3 +1,5 @@
+version = "2"
+
 [logging]
 threshold = "error"