Add events target_type to the user contribution events API endpoints

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Problem to solve

According to our documentation:

Events associated with epics are not available using API.

One cannot retrieve event related to Epics as of now. The only available target types are listed here: https://docs.gitlab.com/ee/api/events.html#target-types

Proposal

Include the epics target_type to the get user contribution event API endpoint and the List currently authenticated user’s events API endpoint.

Edited by 🤖 GitLab Bot 🤖