Add "member_events" group hook to be set via REST API

What does this MR do and why?

Allow member_events to be set via group hook REST API [1][2].

Note:

• I'm not too sure about the variable ordering convention, so I just put the member_events as the last record after the other *_events
• Group hook UI is under "ee", so I requested a trial license to test the functionality end-to-end in my local

References:

• [1] #291169 (closed)
• [2] https://docs.gitlab.com/ee/api/groups.html#add-group-hook
• [3] !90310 (merged)

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Before	After
List:	List:
Edit:	Edit:
Show:	Show:
Create:	Create:

How to set up and validate locally

1. Create new access token in http://127.0.0.1:3000/-/user_settings/personal_access_tokens
2. Create new group hook (considering the group name is flightjs), in http://127.0.0.1:3000/groups/flightjs/-/hooks. Click "Add new webhook"
3. Fill in the details then click "Add webhook"
4. Call REST API to test changing / viewing the "Member Events" value:

# Edit
curl -s -XPUT http://127.0.0.1:3000/api/v4/groups/33/hooks/3\?access_token\=your-access-token \
    --header 'Content-Type: application/json' \
    --data '{"member_events":true}'

# List
curl -s http://127.0.0.1:3000/api/v4/groups/33/hooks\?access_token\=your-access-token

# Show
curl -s http://127.0.0.1:3000/api/v4/groups/33/hooks/3\?access_token\=your-access-token

# Create
curl -s http://127.0.0.1:3000/api/v4/groups/33/hooks\?access_token\=your-access-token \
    --header 'Content-Type: application/json' \
    --data '{"url":"https://example.com/post","member_events":true}'

Related to #291169 (closed)