src/campaign-file: Set Cache-Control

This would allow for images to be cached on the client.
Age of cache  is set to 6 months.

Test:
1. Open Browser's devtools
2. Go to network tab
3. Refresh page
4. Campaign files should be loaded through disk cache

apps/api/src/campaign-file/campaign-file.controller.ts

@@ -12,6 +12,7 @@ import {
   Body,
   Inject,
   forwardRef,
+  Header,
 } from '@nestjs/common'
 import { FilesInterceptor } from '@nestjs/platform-express'
 import { UseInterceptors, UploadedFiles } from '@nestjs/common'
@@ -78,6 +79,7 @@ export class CampaignFileController {
 
   @Get(':id')
   @Public()
+  @Header('Cache-Control', 'public, s-maxage=15552000, stale-while-revalidate=15552000, immutable')
   async findOne(
     @Param('id') id: string,
     @Response({ passthrough: true }) res,