[Feature] Allow defining upload server endpoint

I have searched the existing feature requests to make sure this is not a duplicate request. • Yes The feature Allow defining an alternative endpoint for uploading assets. This is beneficial to Free Cloudflare users and others who do not want to expose their entire Immich stack to the world. Example: upload.domain.com is defined somewhere (within the admin setting or on the local client [app/web/etc..].. 1. For Free Cloudflare users: upload.domain.com can be DNS-only proxied allowing getting around the 100MB upload limit while still protecting the front-end and other endpoints behind Cloudflare. 2. For people who don't want to expose their entire stack: They can expose only upload.domain.com (proxied to /api/upload... elsewhere) and allow clients to upload assets without being on VPNs, etc.. Beneficial for background uploads as well since some VPNs won't connect fast enough on-demand (or ever) before uploading. In my opinion the Immich server shou…