Implement proxy protocol for the server side

[bdraco]

Will be needed before we can flip the flag at https://github.com/NabuCasa/snitun/pull/303/files#diff-962be76744351102b2907e3aab99d5a45bc091cac1cc65f53f0fe2db9033c47fR30

Will need to hide it behind an ENV var since we don't want to read/accept proxy protocol on the server side unless its enabled

[ludeeus]

There was some work started in #227 for that, but it seems to have halted.