Way to set mirror interval via API?

[rcarmo]

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Description

There seems to be no way to set the mirror interval using the API. I can see mirror: true when I GET /repos/{owner}/{repo}, and I see the Web UI POSTing to /user/repo/settings, but there is no corresponding API call...
...

[6543]

we can either add it as option to repo endpoints or make a new endpoint for mirrors only