add ServerConfig with env variables

src/config.rs

@@ -0,0 +1,10 @@
+use bon::Builder;
+
+#[derive(Debug, Builder)]
+pub struct ServerConfig {
+    #[builder(default = String::from("0.0.0.0"))]
+    pub host: String,
+    #[builder(default = 6171)]
+    pub port: u16,
+    pub max_connections: Option<usize>,
+}