Add jh_custom_labels config support epic

What does this MR do and why?

• related issue https://jihulab.com/gitlab-cn/gitlab/-/issues/3579
• related MR !1583 (merged)

Add jh_custom_labels config support epic

example:

 development:
   jh_custom_labels:
     issue:
       - { scope: "priority",
           required: true }
     epic:
       - { scope: "priority",
           required: true }

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