Use exist well-established configurion parser

[sagilio]

the model file is a standard ini format, we can use the existing configuration interface to more easily to integrate to .NET.

ref: https://www.nuget.org/packages/Microsoft.Extensions.Configuration.Ini #263 #258

[casbin-bot]

@sagilio @xcaptain @huazhikui

[hsluoyz]

@sociometry

[hsluoyz]

@sagilio will this std lib API align with our commenting style? https://casbin.org/docs/en/syntax-for-models#docsNav:~:text=A%20model%20CONF%20can%20contain%20comments.%20The%20comments%20start%20with%20%23%2C%20and%20%23%20will%20comment%20the%20rest%20of%20the%20line.

[sagilio]

@hsluoyz Yes, they have the same style.