Backport loading state from deprecated button to current button component

Our GlDeprecatedButton component support a loading state: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-deprecated-button--loading-button

But the newer component does not: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-button--default

Let's backport this feature to the current component.

/cc @lauraMon @shampton