@ngrx/component: Add suspense template as input of LetDirective

[markostanimirovic]

Add new input to the LetDirective that accepts suspense template:

@Component({
  selector: 'ngrx-posts',
  template: `
    <ul *ngrxLet="posts$ as posts; suspenseTpl: loading">
      <li *ngFor="let post of posts">{{ post.name }}</li>
    </ul>
    <ng-template #loading>
      <p>Loading...</p>
    </ng-template>
  `
})
export class PostsComponent {
  readonly posts$ = this.postsService.getPosts();

  constructor(private readonly postsService: PostsService) {}
}

The suspense template should be displayed when passed Observable is in a suspense state.