Added C++ ReactContext for Module Initialize method

[vmoroz]

As we work on the C++ TurboModules, we want this design to be applicable for the generic C++ code and not just C++/WinRT. During the latest TurboModule design review with @acoates-ms he has noticed that the only required use of the C++/WinRT interfaces in the user-written TurboModule code is the new Initialize method with the REACT_INIT attribute. In all other places we already use abstractions such as ReactConstantProvider, ReactPromise, ReactError, etc.

In this change we add the new ReactContext into the Microsoft.ReactNative.Cxx shared code:

• It wraps up the IReactContext
• It adds convenience templated methods that directly accept JS function or event arguments. Internally they create the JSValueArgWriter delegate with help of the new MakeJSValueArgWriter method.
• Its use for the Initialize method is similar to the use of the ReactConstantProvider for the constant provider methods.

Note that it is a breaking change for any code that already started to use the REACT_INIT attribute for Native Modules. After this PR is merged, all code must be changed to use the ReactContext struct instead of the IReactContext interface. The REACT_INIT is a new attribute in RNW 0.62 and the change must have low impact on existing code.

Microsoft Reviewers: Open in CodeFlow

[NickGerleman]

Going to merge this in so we can have it in the initial 0.62 preview and not deal with it ad a breaking change.

[ghost]

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