Extract extension method from mr_widget_options

(EE Port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24891)

What does this MR do?

This creates a getServiceEndpoints method in CE mr_widget_options which is extended in EE mr_widget_options. This fixes the CE / EE diff of this file.

Why is this a big deal now?

For #1979 (closed), there are additional api endpoints which will be needed.

What are the relevant issue numbers?

• #1979 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer