[7.x] Cast returned client identifier value to string

[strongholdmedia]

Fixes #1089.

Will benefit anyone who wishes to switch to native prepared statements for performance or other reasons.

Since it should be 100% backwards compatible, please let me know what kinds of tests would one need for such cases.

[strongholdmedia]

As a side note, please note that I removed the entity trait reference to prevent future ambiguity.
This is due to it has an impedance with the client interface.
Whilst EntityTrait has a type hint of mixed for all entities, ClientInterface suggests identifiers should be strings outright.

If this is something that is problematic, please let me know. Doing so would not cause any problems in execution, but may fail phpstan checks later on when trait support will be sufficiently advanced. :)