Add support for CoreCLR builds

[anonymous2585]

Pull Request Template

Description

This PR allows Reflex to work in CoreCLR bulids (Unity 6000.7.0a2 and above).

I used the IL2CPPActivatorFactory for CoreCLR builds, in all my tests it works. Maybe in the future, we will need to change it by a dedicated ActivatorFactory.

Fixes #152

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Tested in Unity 6000.7.0a2 with this project
• Tested in Unity 6000.7.0a2 in my bigger project
• Tested in Unity 6000.0.69f1 with this project that there is no changes

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked my code and corrected any misspellings
• I have checked that Reflex.GettingStarted still runs nicely