Replace Viper as configuration manager

[davidmasek]

Viper as a configuration manager has it's problems, see https://github.com/knadh/koanf?tab=readme-ov-file#alternative-to-viper for some discussion.

For our use-case it is especially relevant that Viper does not handle keys without content well (i.e. it ignores them). But for Beacon service without configuration is still significant - since we want to know the name of the service that should be monitored even if no config is provided (we just supply the default one). See:

• Keys with nil values are ignored spf13/viper#819
• Viper drops empty tables from config file when calling viper.Unmarshal(&struct{}) spf13/viper#406

It seems like we need different solutions. Alternatives:

• https://github.com/knadh/koanf
• roll own

Since we require somewhat special parsing anyway (dynamic content for the config file, we already parse services pretty much on our own) I think it would make sense to just create our own parser with https://github.com/go-yaml/yaml .

1. Create a module to store config functionality. config seems like a clear choice but would shadow variables of this name. configurer would help with this, but better name would be welcome.
2. Implement the functionality to parse YAML file.
3. Implement the functionality to overwrite values with ENV variables.
4. Remove Viper and change usage to new configurer thingy in the codebase.