Support variables in the `{% now %}` template tag

### Code of Conduct

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

### Feature Description

Allow passing a variable from the template context into the `{% now %}` template tag to allow dynamically choosing the formatting.

### Problem

[Django's `{% now %}` template tag](https://docs.djangoproject.com/en/6.0/ref/templates/builtins/#now) currently only supports a hardcoded string format defined directly in the template, or one of a small number of specific settings. By supporting template variables, it would be possible to change the `format` dynamically (for example, based on a user's locale).

### Request or proposal

proposal

### Additional Details

[As I explain in my blog post](https://blog.lilyf.org/posts/django-template-quirks-3/#the--now--tag), the `now` tag has some very weird behaviour caused by assuming the `format` will always be a string. By expanding to support variables, we can also clean up these weird edge-cases by raising a `TemplateSyntaxError`.

### Implementation Suggestions

Several other template tags support variables, so it should be fairly easy to reuse their implementation of this.