Add "is one of" (IN) operator for numeric fields

I have had this change in one of my projects for a while, thought to try getting in contrib since it is useful in some cases.

Opened a MR, but leaving in "Needs work", since it at least surely needs some tests.

Thanks for creating this MR!
Does look like it might be useful for others, yes. You’re also right that it definitely needs some tests, but probably also some description for users on how to input the values. (Comma-separated doesn’t seem self-evident.) Furthermore, it seems to me like your code might lead to problems, depending on the backend, when a user includes whitespace with the commas (e.g., 1, 2 instead of 1,2).

I’ll first wait for others to say that this would be helpful for them, and maybe to continue work here. But definitely something that could be added, if enough people find it useful. Thanks again!