feat: env variable substitutions

lib/utils/envvar_utils.dart

@@ -35,3 +35,53 @@ List<EnvironmentVariableModel> getEnvironmentSecrets(
           (removeEmptyModels ? element != kEnvironmentSecretEmptyModel : true))
       .toList();
 }
+
+String? substituteVariables(
+    String? input,
+    Map<String?, List<EnvironmentVariableModel>> envMap,
+    String? activeEnvironmentId) {
+  if (input == null) return null;
+
+  final Map<String, String> combinedMap = {};
+  final activeEnv = envMap[activeEnvironmentId] ?? [];
+  final globalEnv = envMap[kGlobalEnvironmentId] ?? [];
+
+  for (var variable in globalEnv) {
+    combinedMap[variable.key] = variable.value;
+  }
+  for (var variable in activeEnv) {
+    combinedMap[variable.key] = variable.value;
+  }
+
+  final regex = RegExp(r'{{(.*?)}}');
+  String result = input.replaceAllMapped(regex, (match) {
+    final key = match.group(1)?.trim() ?? '';
+    return combinedMap[key] ?? '';
+  });
+
+  return result;
+}
+
+HttpRequestModel substituteHttpRequestModel(
+    HttpRequestModel httpRequestModel,
+    Map<String?, List<EnvironmentVariableModel>> envMap,
+    String? activeEnvironmentId) {
+  var newRequestModel = httpRequestModel.copyWith(
+    url: substituteVariables(
+      httpRequestModel.url,
+      envMap,
+      activeEnvironmentId,
+    )!,
+    headers: httpRequestModel.headers?.map((header) {
+      return header.copyWith(
+        value: substituteVariables(header.value, envMap, activeEnvironmentId),
+      );
+    }).toList(),
+    params: httpRequestModel.params?.map((param) {
+      return param.copyWith(
+        value: substituteVariables(param.value, envMap, activeEnvironmentId),
+      );
+    }).toList(),
+  );
+  return newRequestModel;
+}