Modify ImplicitConversionToBinaryContentVisitor to allow for exception list

[christothes]

We have this [visitor[(https://github.com/openai/openai-dotnet/blob/main/codegen/generator/src/Visitors/ImplicitConversionToBinaryContentVisitor.cs) which removes the generated BinaryContent implicit conversion because it wasn't useful for any of our models:

However, this is definitely useful for our protocol models. Let's modify the visitor so that we can keep a list of types that we can "opt-out" of the visitor, so that this code gets generated instead of having to keep it as custom code.

[joseharriaga]

Fix: #865