Vendure Admin Regenerate cache button

regenereate-cache.module.ts

```ts
import { NgModule } from '@angular/core';
import { SharedModule, addActionBarItem } from '@vendure/admin-ui/core';
import { take } from 'rxjs/operators';

@NgModule({
  imports: [ SharedModule ],
  providers: [
    addActionBarItem({
      id: 'isr-regenerate-cache',
      label: 'Regenerate cache',
      locationId: 'product-detail',
      buttonStyle: 'outline',
      onClick: (event, ctx) => {
        const productId = ctx.route.snapshot.params.id;
        const product$ = ctx.dataService.product.getProduct(productId).single$;
        product$.pipe(take(1)).subscribe(res => {
          const clientAppPath = `http://localhost:4200/api/invalidate?secret=MY_TOKEN&urlToInvalidate=`;
          const cachePath = `/product/${res.product?.slug}`;
          fetch(clientAppPath + cachePath).then(r => r.json()).then(res => {
            console.log(res);
            alert(res.message)
          });
        })}
    }),
  ]
})
export class ISRClearCacheModule {}
```

### Usage in vendure-config.ts
```ts
AdminUiPlugin.init({
    route: 'admin',
    port: 3002,
    app: compileUiExtensions({
        outputPath: path.join(__dirname, '../admin-ui'),
        extensions: [{
            extensionPath: path.join(__dirname, 'ui-extensions'),
            ngModules: [{
            type: 'shared',
            ngModuleFileName: 'regenerate-cache.module.ts',
            ngModuleName: 'ISRClearCacheModule',
            }],
        }],
    }),
}),
```