This PR addresses the following situation: we have a core component that injects a list of other components:

public class MyService {
    public MyService(List<A> listOfA) {...}
}

Where A is extensible:

@Extensible
class A {...}

These A components don't add any additional logic but simply provide configuration.

So instead of requiring them to extend A, the preferred approach is to create instances of A that can be registered with the injector:

@Extension
public static final A INSTANCE_1 = new A(...)

Let's look at this with a concrete example:
ThreadPool is service, which injects a collection of FixedExecutorBuilderSpec (only fixed for now) (in the future they should replace ExecutorBuilder)

FixedExecutorBuilderSpec provides all necessary information to create executor that will be used in ThreadPool.

FixedExecutorBuilderSpec is marked with annotation @Extensible

In Downsampling plugin we create an instance of FixedExecutorBuilderSpec and mark it with annotation @Extension:

    @Extension
    public static final FixedExecutorBuilderSpec DOWNSAMPLE_EXECUTOR = new FixedExecutorBuilderSpec(
        DOWNSAMPLE_TASK_THREAD_POOL_NAME,
        settings -> ThreadPool.oneEighthAllocatedProcessors(EsExecutors.allocatedProcessors(settings)),
        DOWNSAMPLE_TASK_THREAD_POOL_QUEUE_SIZE,
        "xpack.downsample.thread_pool"
    );