Add new NODATA blocking mode

[DL6ER]

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10

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This PR add another blocking mode that is reply to each blocked request with no data, i.e., status code NOERROR and empty A and AAAA records. It is unclear if this is advantageous, however, users may want to play around with it.

This template was created based on the work of udemy-dl.

[AzureMarker]

A PR should also be made to https://github.com/pi-hole/docs/

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-v4-2-available-with-shared-memory-new-blocking-modes-and-more/16763/1