Add a spread option

[jimaek]

It would be useful to force a different probe selection algorithm to spread the selection to as many unique locations as possible.

If a user selects "world+spread" we select at least 1 probe per country and multiple ones if the limit allows. If user selects "germany+spread" we select 1 probe per ASN. For ASNs we can do 1 probe per city within that ASN.

[inventarSarah]

That would be very useful -- I noticed that when I test from "Europe", the API loves to pick Falkenstein as the location.

I created a monitor in Checkmate and set "Europe" as the location -- as a user, I would expect Globalping to test from different locations in that region. However, I currently have 23 uptime checks, all from Falkenstein.