feat: change feature flag schema to store default value (#742)

This updates the feature flag schema to store the default value of the
flag.

---
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under the terms of the Apache-2.0 license

Author: vivian12300

packages/@aws-cdk/cloud-assembly-schema/lib/cloud-assembly/artifact-schema.ts

@@ -271,6 +271,13 @@ export interface FeatureFlag {
    * @default - No description
    */
   readonly explanation?: string;
+
+  /**
+   * The value of the flag if it is unconfigured
+   *
+   * @default - No value
+   */
+  readonly unconfiguredBehavesLike?: { [key: string]: any };
 }
 
 /**