Add validation to prevent user creation with duplicate email

[rajaSahil]

What this PR does / why we need it:

This PR introduces server-side validation to reject duplicate user objects based on the email field.

Add validation at the Kubernetes API level to ensure:

• Email address uniqueness across all user objects

Which issue(s) this PR fixes:

Fixes #15092

What type of PR is this? /kind bug

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

Add validation to prevent user creation with duplicate email

Documentation:

NONE