Add ConditionalAttribute to codegen attributes

[heaths]

The various codegen attributes don't need to ship in productio assemblies. They cause little harm, but still get enumerated and, as with #3261, can even cause (admitted small) perf hits.

We should put ConditionalAttributes on these attributes. .NET does that for SuppressMessageAttribute, for example. Only when CODE_ANALYSIS is passed - during code analysis - is the attribute even included. It's elided for normally compiled code. We could do the same with a CODEGEN symbol or something that autorest.csharp itself could pass, such that any scripts/MSBuild target - however it's called - uses it.

This would make the code a little leaner.

See Azure/azure-sdk-for-net#35376 (comment) for context.