Allow adding _in validators to attr classes

[mariajgrimaldi]

As per my question [here](Here's the original thread: https://openedx.slack.com/archives/C03SDBSFUUB/p1714139377765319):

There's a new event in progress here, which included a status str attribute with a validator: https://github.com/openedx/openedx-events/pull/303/commits/8558418e230edcf29687661bcdb3def5bb1a58bc#diff-5cf3a0b26961626ce[…]a35d078cb9c62438fa3R521. Without this modification tests would fail because of the validator, but the change was too specific for this use case, so the team implementing the event decided to drop the validator altogether. The thing is, we should be able to add this kind of stuff, so should we implement a workaround for it?

I'm opening this issue so we can go back to it in the future.