Add InertiaMeta for automatic serialization

[BrandonShar]

This PR adds the concept of InertiaMeta as suggested by @scajanus in #65

class User(models.Model):
  name = models.CharField(max_length=255)
  password = models.CharField(max_length=255)
  created_at = models.DateField(auto_now_add=True)

  class InertiaMeta:
    fields = ('name', 'created_at')

Will result in the props output of a User instance being

{
  "name": "Brandon",
  "created_at": "2025-01-25T10:13:01"
}

Because of the implementation, InertiaMeta can also be used on any class, not just models!