Refactor to `configService` to read & write `Config`

`configService` can save and load `Config` using a strategy of `configEncoder`
and `configDecoder`. This pave the path for swapping out `tomlConfigEncoder`
and `tomlConfigDecoder` with `yamlConfigEncoder` and `yamlConfigDecoder`.

`Configs` also has been renamed to `Config`.

/cc @mislav 
The decoder and encoder components feel too lightweight and don't warrant being extracted to source files of their own. I would be more in favor of config "reader" and "writer" APIs that would use either TOML/YAML internally but also have more responsibilities such as reading/writing to the config file.