First step to target fully source gen implementation

[kant2002]

I extract simple model for Refit client representation withing source gen That to allow facilitate process

• Parse Syntax (done by Roslyn itslef)
• Build internal model
• Write based on model only, so logic would be as simple as possible

Also, that allows write more robust test if we test just internal model extraction from symbols. Otherwise, if we will capture generated output, if we decide improve generation it will produce a lot of churn. I do not against churn like that, but not sure that would be appropriate.

Related to #1389