feat: add LazyJsonString class (smithy-lang#899)

This commit adds the `LazyJsonString` class that supports holding a
String with JSON contents, deserializing to JSON, and being built
from JSON like content.

packages/smithy-client/src/index.ts

@@ -3,3 +3,4 @@ export * from "./command";
 export * from "./document-type";
 export * from "./exception";
 export * from "./isa";
+export * from "./lazy-json";

packages/smithy-client/src/lazy-json.ts

@@ -0,0 +1,21 @@
+/**
+ * Lazy String holder for JSON typed contents.
+ */
+export class LazyJsonString extends String {
+  deserializeJSON(): any {
+    return JSON.parse(super.toString());
+  }
+
+  toJSON(): string {
+    return super.toString();
+  }
+
+  static fromObject(object: any): LazyJsonString {
+    if (object instanceof LazyJsonString) {
+      return object;
+    } else if (object instanceof String || typeof object === 'string') {
+      return new LazyJsonString(object);
+    }
+    return new LazyJsonString(JSON.stringify(object));
+  }
+}