Add secondary button to gl-modal component

What does this MR do?

Add options for a secondary action to the gl-modal Vue component.

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/42170, https://gitlab.com/gitlab-org/gitlab-ce/issues/28849