Add auth check to import github api endpoints

What does this MR do and why?

Fix for follow-up issue #377724 (closed)

Adds authentication check for /api/v4/import/github/cancel and /api/v4/import/github endpoints

Screenshots or screen recordings

POST /api/v4/import/github

import

POST /api/v4/import/github/cancel

cancel

How to set up and validate locally

For import:

1. Send POST /api/v4/import/github without provided PAT in headers

For cancel:

1. Trigger project import using POST /api/v4/import/github
2. Send POST /api/v4/import/github/cancel without provided PAT in headers

MR acceptance checklist

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