[9.x] Add hasUser method to Guard contract

[mpyw]

The following PR by @TBlindaruk has been previously rejected at v5.8.

• [5.8] Added hasUser method to Guard contract; #25629

However, we are now at a new stage with many changes, such as introducing Contract on Query Builder implementation. I decided that it would be ready to resubmit this PR now. It is useful in some situations that knowing whether user() causes side effects in advance.

Note that we don't have to add setProvider() and getProvider() because they are mainly for internal use.

Breaking Changes

• All custom Guard without GuardHelpers should explicitly implement hasUser() from now on.
  • Most Guard implementation which uses property caching should return ! is_null($this->user)
  • Guard without any caches should always return false

[driesvints]

I think we can rename this to a more applicable method like checkSilently and then add that as an alias on the GuardHelpers trait.

I do have to say that ideally, I wish check didn't produce any side effects. But it's too much of a breaking change probably to change that now.

[mpyw]

@taylorotwell Hi! Do you have any ideas about renaming hasUser()?