Add endpoint to search user by email (admin only)

[lindaeskilsson]

GET /users/search?email=
Only admins should be able to search for users by email.
Requires Spring Security to be implemented first so the
endpoint can be secured with @PreAuthorize("hasRole('ADMIN')")

Reason: Admins need to be able to look up a specific user by email,
for example to manage accounts, reset passwords, or verify identity.
The endpoint is not exposed now since there is no way to restrict
access to admins only without Spring Security in place.