Add resource events to Import/Export

What does this MR do and why?

This MR:

• Adds resource_state_events information to Epics in Group Import/Export in order to show opened/closed events accurately on epics page
• Adds resource_state_events information to Issues/MRs in Project Import/Export, same as above
• Adds resource_milestone_events information to Issues/MRs in Project Import/Export, in order to show milestone related events on issue/mr pages

Mentions #291983 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Create a group and a project in it
2. Create 1 epic, 1 issue & 1 MR
3. Open/close a few times the epic/issue/MR to generate resource_state_events
4. Create a new milestone and assign/deassign it to issue/MR to generate resource_state_events
5. Export group & project (each done separately in General -> Advanced settings)
6. Download exported group & project tar.gz files
7. Import the group using exported group tar.gz file. Verify epic has preserved state events
8. Import the project using exported project tar.gz file. Verify imported issue/MR has preserved state & milestone 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.