[Pleroma.Uploaders.Local]: Add configuration for custom url path

One use-case being an external caching proxy
2 jobs for feature/custom_media_url in 4 minutes and 55 seconds (queued for 1 minute and 51 seconds)
Status Job ID Name Coverage
  Lint
passed #7428
lint

00:02:07

 
  Test
passed #7429
unit-testing

00:02:48