Fill default configuration values when parsing the configuration YAML

[tale]

Right now, I'm doing a very simple YAML parsing on the configuration file, but not filling in missing values with their defaults. This could be fixed with an idiomatic configuration API, however for now the solution would be filling in the default values from the upstream.

https://github.com/juanfont/headscale/blob/main/hscontrol/types/config.go