Rest API 접근을 위한 HttpClientModule 등록

src/app/app.module.ts 에 import 추가

import { FormsModule, ReactiveFormsModule } from '@angular/forms';
import { HttpClientModule } from '@angular/common/http';

imports: [
  BrowserModule,
  FormsModule,
  ReactiveFormsModule,
  HttpClientModule,
  AppRoutingModule
],