Implement validation for PSK and PSK identity fields in Frontend

It will by nice to have validation for PSK and PSK identity fields in Frontend.

For now we only have this warning in documentation:

Each PSK identity must be paired with only one value. It is the user's responsibility to ensure that there are no two PSKs with the same identity string but different values. Failing to do so may lead to unpredictable disruptions of communication between Zabbix components using PSKs with this PSK identity string

But since this information stored in configuration cache and database - we can check what user want to insert and  don't allow user use same  PSK and PSK identity for more then one host.