[8.x][Security] Implement case insensitive function-based unique index.

[yajra]

A security patch for unique constraint to enforce case insensitive matching:

Example:

Before PR

Both users will be inserted without throwing a constraint error.

users: email (unique)

User::create(['email' => 'admin@example.com']);
User::create(['email' => 'Admin@example.com']);

After PR

User::create(['email' => 'admin@example.com']);
User::create(['email' => 'Admin@example.com']);

Error Message : ORA-00001: unique constraint (SCHEMA.USERS_EMAIL_UK) violated

NOTE

This patch only works on projects with fresh migrations. For existing projects, you need to create the unique index manually.