Add before_send hook support

[marandaneto]

Summary

Add a before_send hook that can mutate or drop assembled events before enqueue.

Why

sdk-specs marks before-send-hook as applicable to both client and server SDKs. It is the standard privacy/filtering interception point across SDKs.

Current status

No config option or hook implementation was found for mutating/dropping events after assembly and before enqueue.

Suggested scope

• Add configuration for one or more before_send callbacks.
• Run the hook after the event has been assembled/enriched but before it is sent to PostHog.Sender.
• Allow callbacks to return a modified event or nil to drop.
• Catch/log hook exceptions without crashing caller code.
• Ensure all public event-producing helpers use the hook consistently.
• Add tests for mutate, drop, and exception paths.

References

• https://github.com/PostHog/sdk-specs/blob/main/openspec/specs/before-send-hook/spec.md

[marandaneto]

#162