feat: add optional language parameter to Valhalla provider (#905)

* feat: add optional language parameter to Valhalla provider

* Refactor Valhalla struct to use options for language

Removed the language field from the Valhalla struct and updated related methods to use options for language settings.

* Formatting fixes

* More formatting fixes

---------

Co-authored-by: Ian Wagner <ian.wagner@stadiamaps.com>

Author: NAKKA-POOJITHA

common/ferrostar/src/routing_adapters/valhalla.rs

@@ -302,7 +302,7 @@ pub struct ValhallaHttpRequestGenerator {
     endpoint_url: String,
     /// The Valhalla costing model to use.
     profile: String,
-    // TODO: Language, units, and other top-level parameters
+
     /// Arbitrary key/value pairs which override the defaults.
     ///
     /// These can contain complex nested structures,
@@ -354,6 +354,14 @@ impl ValhallaHttpRequestGenerator {
             options,
         }
     }
+    /// Sets the language for routing instructions (e.g., "de", "en-US").
+    ///
+    /// This is a convenience helper that populates the internal options map.
+    pub fn with_language(mut self, language: impl Into<String>) -> Self {
+        self.options
+            .insert("language".to_string(), serde_json::json!(language.into()));
+        self
+    }
 
     /// Creates a new Valhalla request generator given an endpoint URL, a profile name,
     /// and options to include in the request JSON.