Add cancel bulk_import api #3292

Description of the problem, including code/CLI snippet

Add high-level api to do POST /bulk_imports/:id/cancel:

• https://docs.gitlab.com/api/bulk_imports/#cancel-a-migration

Currently doesn't seem to be present:

• https://github.com/python-gitlab/python-gitlab/blob/b70a2eb9ff5195f123c41cf1d631ff151ce089f1/gitlab/v4/objects/bulk_imports.py

Specifications

• python-gitlab version: 6.5.0
• Gitlab server version (gitlab.com): 18.6.0-pre 6b0bede0a5e