Batch validations

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

### AI Policy

- [x] I agree to follow this project's AI Policy, or I agree that AI was not used while creating this issue.

### Is your feature request related to a problem? Please describe.

Changes support bulk creates by implementing the `batch_change` callback, but validations don't support something similar.

In practice, this can be worked around by creating a change module that implements `batch_change` and using it as if it was a validation, but this feels a little odd compared to a builtin solution with validations.

### Describe the solution you'd like

It would be nice for there to be a `batch_validate` callback (and friends) on `Ash.Resource.Validation`.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start with Ash.Resource.Validation and compare the requested batch_validate callback with the existing batch_change callback behavior described in the issue. Trace how validations are defined and invoked, then establish the expected behavior for batch validation and its related callbacks; the work is done when bulk creates can use the builtin validation support consistently.

Written by the indexing model from the issue text.