Introduce ContextThreadFactory

[philsttr]

Introduce ContextThreadFactory to propagate context to new Threads created by a delegate ThreadFactory.

The new ContextThreadFactory class was loosely modeled after the existing ContextExecutorService class. They serve essentially the same purpose, but ContextThreadFactory wraps a ThreadFactory instead of an ExecutorService.

See this related comment

Example usage with Java 25's Structured Concurrency preview:

public class RobotAssemblyService {

    // Wrap a ThreadFactory with a ContextThreadFactory that propagates context
    private final ThreadFactory threadFactory = ContextThreadFactory.wrap(
            Thread.ofVirtual().name("robot-assembly-", 0).factory(),
            ContextSnapshotFactory.builder().build());

    public Robot assemble() throws InterruptedException {
        try (var scope = StructuredTaskScope.open(
                awaitAllSuccessfulOrThrow(),
                configuration -> configuration
                        .withName("robot-assemble")
                        .withTimeout(Duration.ofSeconds(2))
                        // use the ContextThreadFactory
                        .withThreadFactory(threadFactory))) {

            Subtask<Arm> armTask = scope.fork(this::assembleArm);
            Subtask<Leg> legTask = scope.fork(this::assembleLeg);

            scope.join();

            Arm arm = armTask.get();
            Leg leg = legTask.get();
            return new Robot(arm, leg);
        } catch (FailedException e) {
            throw new RobotAssemblyException(e.getCause());
        } catch (TimeoutException e) {
            throw new RobotAssemblyException(e);
        }
    }

    // ...
}

[philsttr]

circleci failure on jdk 11 looks unrelated to my change.