[Proposal]: Start to support private fields/properties

[alexinea]

In earlier versions of Leo, we supported private properties or fields. But starting from the current version, Leo will no longer support access to private properties or fields by default.

For this, we will try several situations to provide users with the convenience of reading and writing private properties or fields.

Now use the following extension methods to support reading and writing of private fields or properties:

type.AllowLeoPrivate();

This method is temporary, as we will implement it in a more elegant and flexible way in the next version or some future version.

https://github.com/night-moon-studio/Leo/blob/master/test/NCallerUT/DictNormalTest.cs#L40