Merge simple and strongly related repo to Casbin.NET repo

[sagilio]

Here are some reasons:

• We will reduce a lot of cost on managing these many libraries.
• It will give a clear experience for users. A user only needs to install the same version of these libraries
• Multiple projects can better share tests and ci/cd workflow changes.
• We can ensure mutual compatibility.

The repo list:

• Casbin.Extensions: https://github.com/casbin-net/extensions

[sagilio]

@hsluoyz @xcaptain What do you think about this issue?

[hsluoyz]

@sagilio good idea, who is working on this?