Add support for case sensitive validation to UniqueFieldValuesConstraint

This is very particularly like 🐛 Unhandled exception when trying to register a duplicate username with different case Needs review .

Was expecting this would also impact username, as it also uses that constraint - but we have a unique key on name for the users table, so instead if causes a fatal (SQL) exception - so still an issue.

Added tests and fix for both cases.

I am new here, can I work on this issue?

I verified for the use cases given above. The fix is working - Error message are displayed 'The email address EMAIL@TEST.COM is already taken.'

Testing Steps :
1. Creating user with Email@test.com
2. Verify when used different cases such as eMail@test.com , EMAIL@test.com - error message must be displayed
3. Verify when used same case Email@test.com with different usernames - - error message must be displayed

Can be moved to RTBC++

Keeping this in need review for further reviews.

Opened an MR and appears to have test failures.

Added a question to the MR.

🐛 Unhandled exception when trying to register a duplicate username with different case Needs review is in so let's repurpose this