Add before-send hook support

[marandaneto]

Summary

Add support for the sdk-spec before-send-hook behavior to the PHP SDK.

Spec

• sdk-specs/openspec/specs/before-send-hook/spec.md

Current evidence

The SDK audit found no before_send / beforeSend option or callback path in PostHog::init, Client, README, or tests.

Expected behavior

• Allow configuring a before-send hook for events before enqueue/send.
• Hook can mutate the assembled event.
• Hook can drop the event by returning null/equivalent.
• Hook exceptions should not crash caller code.
• Add tests covering mutation, drop, and exception handling.