feat: add support for global URI variables

lib/src/core/consumed_thing.dart

@@ -266,15 +266,20 @@ class ConsumedThing implements scripting_api.ConsumedThing {
       Form form, InteractionOptions? options) {
     final hrefUriVariables = _filterUriVariables(form.href);
     final optionUriVariables = options?.uriVariables;
-    final affordanceUriVariables = interactionAffordance.uriVariables;
+
+    // Use global URI variables by default and override them with
+    // affordance-level variables, if any
+    final Map<String, Object?> affordanceUriVariables = {}
+      ..addAll(thingDescription.uriVariables ?? {})
+      ..addAll(interactionAffordance.uriVariables ?? {});
 
     if (hrefUriVariables.isEmpty) {
       // The href uses no uriVariables, therefore we can abort all further
       // checks.
       return form.href;
     }
 
-    if (affordanceUriVariables == null) {
+    if (affordanceUriVariables.isEmpty) {
       throw UriVariableException("The Form href ${form.href} contains URI "
           "variables but the TD does not provide a uriVariables definition.");
     }

lib/src/definitions/thing_description.dart

@@ -72,6 +72,11 @@ class ThingDescription {
   /// A map of [SecurityScheme]s that can be used for secure communication.
   final Map<String, SecurityScheme> securityDefinitions = {};
 
+  /// URI template variables as defined in [RFC 6570].
+  ///
+  /// [RFC 6570]: http://tools.ietf.org/html/rfc6570
+  Map<String, Object?>? uriVariables;
+
   /// Creates a [ThingDescription] from a [rawThingDescription] JSON [String].
   ThingDescription(this.rawThingDescription) {
     parseThingDescription(rawThingDescription!);
@@ -133,6 +138,10 @@ class ThingDescription {
     if (links is List<dynamic>) {
       _parseLinks(links);
     }
+    final dynamic jsonUriVariables = json["uriVariables"];
+    if (jsonUriVariables is Map<String, dynamic>) {
+      uriVariables = jsonUriVariables;
+    }
   }
 
   // TODO(JKRhb): Refactor