feat: add VerificationAttempt model to verify_student application

This commits adds a VerificationAttempt model to store implementation and provider agnostic information about identity verification attempts in the platform.

lms/djangoapps/verify_student/models.py

@@ -1213,4 +1213,5 @@ class VerificationAttempt(TimeStampedModel):
     expiration_datetime = models.DateTimeField(
         null=True,
         blank=True,
+        db_index=True,
     )