New setting: turn off logging of added sources

[pjanck]

While I appreciate the log, I would prefer to have an option to turn it off and log myself if I want.

SourceGenerator.Foundations/src/SourceGenerator.Foundations.Contracts/SgfSourceProductionContext.cs

Line 43 in 2a9f13a

m_generator.Logger.Information($" SourceAdded: {SourceCount}. {hintName}");

It pollutes the output - I am generating 3 x 3500 files :)