Provide SimpleAsyncTaskExecutorBuilder to build SimpleAsyncTaskExecutors

[wilkinsona]

Both TaskExecutorBuilder and TaskExecutorCustomizer are aimed at ThreadPoolTaskExecutor. We should review them with virtual threads in mind. I suspect we may end up with some new classes that are aimed at SimpleAsyncTaskExecutor which provides virtual thread support. If we take that route, we may want to rename the existing builder and customiser to ThreadPoolTaskExecutorBuilder and ThreadPoolTaskExecutorCustomizer.

[mhalbritter]

The rename of the existing builder is handled in #36637.

[mhalbritter]

There's now a SimpleAsyncTaskExecutorBuilder in the context to build SimpleAsyncTaskExecutors. When virtual threads are enabled, the SimpleAsyncTaskExecutorBuilder will be auto-configured to use them. One can use SimpleAsyncTaskExecutorCustomizer to customize the built SimpleAsyncTaskExecutors.