✨ feat(config): introduce client configuration

- adds `ClientConfig` struct for managing Gmail client settings
- supports loading credentials from environment variables or a JSON file
- provides methods to access the application secret and configuration root

src/client_config.rs

@@ -0,0 +1,67 @@
+use std::{fs, path::PathBuf};
+
+use config::Config;
+use google_gmail1::yup_oauth2::{ApplicationSecret, ConsoleApplicationSecret};
+
+use crate::Result;
+
+mod config_root;
+
+use config_root::ConfigRoot;
+
+/// Configuration for the gmail client
+#[derive(Debug)]
+pub struct ClientConfig {
+    secret: ApplicationSecret,
+    config_root: ConfigRoot,
+}
+
+impl ClientConfig {
+    /// Create new configuration from configuration
+    pub fn new_from_configuration(configs: Config) -> Result<Self> {
+        let root = configs.get_string("config_root")?;
+        let config_root = ConfigRoot::parse(&root);
+
+        let secret = if let Ok(client_id) = configs.get_string("client_id")
+            && let Ok(client_secret) = configs.get_string("client_secret")
+            && let Ok(token_uri) = configs.get_string("token_uri")
+            && let Ok(auth_uri) = configs.get_string("auth_uri")
+        {
+            ApplicationSecret {
+                client_id,
+                client_secret,
+                token_uri,
+                auth_uri,
+                project_id: None,
+                redirect_uris: Vec::new(),
+                client_email: None,
+                auth_provider_x509_cert_url: None,
+                client_x509_cert_url: None,
+            }
+        } else {
+            let credential_file = configs.get_string("credential_file")?;
+            let path = PathBuf::from(root).join(credential_file);
+            let json_str = fs::read_to_string(path).expect("could not read path");
+
+            let console: ConsoleApplicationSecret =
+                serde_json::from_str(&json_str).expect("could not convert to struct");
+
+            console.installed.unwrap()
+        };
+
+        Ok(ClientConfig {
+            config_root,
+            secret,
+        })
+    }
+
+    /// Report a reference to the secret.
+    pub fn secret(&self) -> &ApplicationSecret {
+        &self.secret
+    }
+
+    /// Report a reference to the config root.
+    pub fn config_root(&self) -> &ConfigRoot {
+        &self.config_root
+    }
+}