feat: Added a user-service, that asserts, that a user is known

[demos/kafka/chat] / src / app / user / user.component.html

<div>
  <label for="name">Name: </label>
  <input id="name" type="text" [formControl]="name">
  <button type="button" (click)="updateName()">Update Name</button>
</div>
<p>Value: {{ name.value }}</p>