Support Asana custom IDs
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### Proposal
Asana has a concept of defining custom ID fields for tasks in a project. This results in a field which is automatically populated by a value in the form of FOO-1234 on every task.
It would be great if GitLab finds these IDs in a commit message, additionally to the GID and URL.
Description: https://help.asana.com/s/article/id-custom-fields?language=en_US
API: https://developers.asana.com/reference/gettaskforcustomid
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