fix(users/auth): Add validation for name & password in registration

[CuriousLearner]

Why was this change necessary?

There was no validation present during registration for password field. If the first_name or last_name will be more than 120 characters, instead of raising error, 500 server error will appear.

How does it address the problem?

It adds max_length validation in first_name and last_name for registration serializer and makes sure to run password validations.

Are there any side effects?

None.