Feature request: Add option to use custom servers.json as the only source of truth

What's the feature

It seems to me gluetun has a lot of hardcoded logic in the server side, e.g. if you specify a custom name as the vpn provider it'll stop working, and it always merges the hardcoded server list even if a user-defined servers.json is supplied. Giving users an option to use only their own servers.json is useful if (take proton vpn as an example):

• You want to exclude some servers (e.g. TOR, free). Currently you only have the option to exclude non-TOR or non-FREE servers
• You want to update servers.json manually (for whatever reason, e.g. default API that gluetun uses isn't working)

Without this option, the merge would bring back those TOR/FREE servers, or other servers that you exclude manually. This option would make management simpler, without having to update both SERVER_HOSTNAMES and servers.json when you only want to connect to a selected set of servers.

Extra information and references

No response