Added `CCodeGenerator.GenerateExtraClassSpecifiers` extension point.
Added `CCodeGenerator.GenerateExtraClassSpecifiers` extension point.

ddobrev/CppSharp,ddobrev/CppSharp,mono/CppSharp,zillemarco/CppSharp,zillemarco/CppSharp,zillemarco/CppSharp,mono/CppSharp,zillemarco/CppSharp,mono/CppSharp,mono/CppSharp,mono/CppSharp,ddobrev/CppSharp,zillemarco/CppSharp,mono/CppSharp,ddobrev/CppSharp,ddobrev/CppSharp