Improvements for graphrag config compatibility

[andresmor-ms]

• Add an option to add a Parser to a field - useful when you need to parse a list from a string.
• Add an ArgSource which you can provide in runtime arguments to the config.
• Fix order of sources. - Add break when the first source matches a field.
• Add silence_error flag when loading files. - useful when the files are optional.
• Add dotenv support for EnvSource