Validate helper version on client_init

[Anilm3]

Description

As a continuation of #3.

The potential, but unlikely, scenario of a rogue outdated helper process running after an upgrade could result in undefined behaviour. To avoid this issue the socket and lock file should be versioned, this would ensure that communication can only be performed between a compatible extension and helper.

This doesn't cover the case in which the actual binary of the helper has not been upgraded.

Since the first extension to launch the helper process opens the socket and lock file for it, validating the helper version on client_init would ensure that an incompatible helper binary still wouldn't result in undefined behaviour, albeit this would prevent appsec from running.