Allow skipping auto-merge on public holidays

[allanlewis]

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I'm looking into enabling auto-merge for some Renovate PRs and I'd like to restrict this to office hours. However, I don't see an option to exclude public holidays: would it be possible to add this? There are lots of public holiday databases around and obviously this would need to be geographic - e.g. I'd like to block auto-merge on UK public holidays.