make "Advanced Server Configurations" be a per server feature

[gkrawiec]

👉 Describe the problem
Right now each provisioned server can be pointed to a different ip and provider, but the advanced features in spin.yml such as port apply to all at once.

👥 Problem evidence & reach

The problem is I am using Spin to provision sandboxed machines in order to use parallel AI agents on them. in MacOS, i am using orbstack which allows sanboxing, but does not allow nested virtualization, so I can have a linux machine, but that linux machine cannot run docker. So instead I use ubuntu LXD. But for this to work I need to use a different port than the default because the virtual machine uses the standard.
But my stage and production machines are on standard ports. So now to deploy sandboxed containers I need to swap spin.yml files.

🥰 Describe the "impact" on users?

as more and more people use AI agents, sandboxing will be a more common situation. This feature will be on more step into making spin trully take care of all deploys

🏆 How to solve this problem

option 1: easy, but annoying. Add a spin.yml file argument to the the spin deploy command. spin deploy -f <>
This would work but the burden is on the user to remember, and also does not allow multi machine deploys/
option 2: make the advanced server configs paramentes of the servers section.