demo-protobuf: Add example using protobufjs decorators

packages/demo-protobuf/src/decorator.spec.ts

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+/* eslint-disable @typescript-eslint/camelcase */
+import { assert } from "@cosmjs/utils";
+
+import { myRegistry } from "./decorator";
+
+describe("decorator demo", () => {
+  it("works with a custom msg", () => {
+    const typeUrl = "/demo.MsgDemo";
+    const MsgDemo = myRegistry.lookupType(typeUrl)!;
+    const TxBody = myRegistry.lookupType("/cosmos.tx.TxBody")!;
+    const Any = myRegistry.lookupType("/google.protobuf.Any")!;
+
+    const msgDemo = MsgDemo.create({
+      example: "Some example text",
+    });
+    const msgDemoBytes = MsgDemo.encode(msgDemo).finish();
+    const msgDemoWrapped = Any.create({
+      type_url: typeUrl,
+      value: msgDemoBytes,
+    });
+    const txBody = TxBody.create({
+      messages: [msgDemoWrapped],
+      memo: "Some memo",
+      timeoutHeight: 9999,
+      extensionOptions: [],
+    });
+    const txBodyBytes = TxBody.encode(txBody).finish();
+
+    const txBodyDecoded = TxBody.decode(txBodyBytes);
+    const msg = txBodyDecoded.messages[0];
+    assert(msg.type_url);
+    assert(msg.value);
+
+    const decoder = myRegistry.lookupType(msg.type_url)!;
+    const msgDemoDecoded = decoder.decode(msg.value);
+    expect(msgDemoDecoded.example).toEqual(msgDemo.example);
+  });
+});

packages/demo-protobuf/src/decorator.ts

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+import { Field, Message, Type } from "protobufjs";
+
+import { Registry } from "./registry";
+
+export const myRegistry = new Registry();
+
+@Type.d("MsgDemo")
+export class MsgDemo extends Message<{}> {
+  @Field.d(1, "string")
+  public readonly example: string = "";
+}
+
+myRegistry.register("/demo.MsgDemo", MsgDemo);

packages/demo-protobuf/types/decorator.d.ts

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+import { Message } from "protobufjs";
+import { Registry } from "./registry";
+export declare const myRegistry: Registry;
+export declare class MsgDemo extends Message<{}> {
+  readonly example: string;
+}