Provide a condition for threading (platform or virtual)

[mhalbritter]

Would be good to have a @ConditionalOnPlatformThreads which is the negation of @ConditionalOnVirtualThreads.

[mhalbritter]

After some consideration, we now use @ConditionalOnThreading(Threading.PLATFORM) for platform threads and @ConditionalOnThreading(Threading.VIRTUAL) for virtual threads.