Support const keyword

Support `const` : https://json-schema.org/understanding-json-schema/reference/const

src/model/openapi31.ts

@@ -301,6 +301,7 @@ export interface SchemaObject extends ISpecificationExtension {
     title?: string;
     multipleOf?: number;
     maximum?: number;
+    const?: string;
     /** @desc In OpenAPI 3.1: number */
     exclusiveMaximum?: number;
     minimum?: number;