Add the resource state events REST API endpoint for epics

What does this MR do and why?

This MR adds support for resource_state_events for epics.

There is no issue for this(yet) but there is a customer request for this based on following slack thread: https://gitlab.slack.com/archives/C72HPNV97/p1662466625278999 and related issue #373293 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Create a group or navigate to an existing one. Note it's ID.
2. Create an epic or navigate to an existing one.
3. Close and reopen the epic.
4. Find the Epic's ID(not IID)
5. access the resource_label_events endpoint got given group and epic:

     http://localhost/api/v4/groups/<group-id>/epics/<epic-id>/resource_label_events

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.