[Gradle] add inputSpecRootDirectory for generate task (#15548) (fix #15483)

Author: borsch

modules/openapi-generator-gradle-plugin/src/main/kotlin/org/openapitools/generator/gradle/plugin/OpenApiGeneratorPlugin.kt

@@ -97,6 +97,7 @@ class OpenApiGeneratorPlugin : Plugin<Project> {
                     generatorName.set(generate.generatorName)
                     outputDir.set(generate.outputDir)
                     inputSpec.set(generate.inputSpec)
+                    inputSpecRootDirectory.set(generate.inputSpecRootDirectory)
                     remoteInputSpec.set(generate.remoteInputSpec)
                     templateDir.set(generate.templateDir)
                     auth.set(generate.auth)

modules/openapi-generator-gradle-plugin/src/main/kotlin/org/openapitools/generator/gradle/plugin/extensions/OpenApiGeneratorGenerateExtension.kt

@@ -54,6 +54,11 @@ open class OpenApiGeneratorGenerateExtension(project: Project) {
      */
     val inputSpec = project.objects.property<String>()
 
+    /**
+     * Local root folder with spec files
+     */
+    val inputSpecRootDirectory = project.objects.property<String>()
+
     /**
      * The remote Open API 2.0/3.x specification URL location.
      */