Mod / Modulus operator as a custom expression

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**Is your feature request related to a problem? Please describe.**
Yes, I wasn't able to do what I wanted. [See here](https://discourse.metabase.com/t/is-there-a-modulus-mod-operator-in-metabase/16747)

> I have a field in my DB which stores an integer between 1-12. This determines the AB test group the user is in: odd means group A, even means group B.
>
> There seems to be no simple way of calculating which group the user is in. Ideally I would create a custom column with the values "ODD" or "EVEN".

**Describe the solution you'd like**
I'd like to see a modulo operator (Python uses '%')

Instead of

```CountIf([Split Test Group Session] = 2 OR [Split Test Group Session] = 4 OR [Split Test Group Session] = 6 OR [Split Test Group Session] = 8 OR [Split Test Group Session] = 10 OR [Split Test Group Session] = 12)```

I would write

```CountIf([Split Test Group Session] % 2 = 2)```

**Describe alternatives you've considered**
It is possible to use workaround with a Custom Column and then using that in the CountIf, but they are ugly.
`case([Field] / 2 = floor([Field] / 2), "Even", "Odd")`

**How important is this feature to you?**
Not super important. But I was very surprised that Metabase was not capable of this. It's a basic logical operator supported by many programming languages.

:arrow_down: Please click the :+1: reaction instead of leaving a `+1` or `update?` comment

Start by tracing how Metabase custom expressions handle arithmetic, comparisons, and CountIf, using the examples in the issue as behavioral references. Done means a modulo operator is accepted in custom expressions and supports the demonstrated odd/even grouping use case, with coverage for representative integer values.

Written by the indexing model from the issue text.