[EuiSteps] Add loading prop to step item for displaying status loading

[formgeist]

Summary

In some scenarios the EuiSteps have a button that will perform some action. In this case in the Logs ML integration, there's a button to create the job. Currently the button is replaced with a spinner and text, but I think there might be a missed opportunity to allow the EuiStep item to have a loading prop to show the spinner and have an optional text in the step.

This could also be used if the page is awaiting a status for the step to present itself as completed or not.

[formgeist]

Thanks for adding this in 👏