[Feature Request] Introduce `user.name` operand

### Summary:
Today you can write rules matching a user's Id (as in `id -u $USERNAME`), but when writing rules it's easier to refer to user names.

Introducing the `user.name` operand can make maintaining rules easier.
Implementation wise, we can at some point (reading, compiling rules) translate it to `user.id` to avoid matching strings.

#### Why?

While the UI helps selecting the Id, there's some issues with this,
- Rules are harder to share across machines since User Ids may change (different distros or user creation order).
- Rules are harder to generate
- (On NixOS we can write rules in the system config, but can't hardcode Ids as those haven't been picked for users/groups that don't exist yet)
- Rules are harder to read.
- I'm not sure why, but some rules don't show a user name, only some Id that looks quite obscure.