Support `before_send` hook like JS/Py/Ruby SDK

[MunifTanjim]

Something like this:

	client, err := posthog.NewWithConfig(
		config.Posthog.APIKey,
		posthog.Config{},
	)

	if err != nil {
		log.Fatal("failed to initialize", "error", err)
	}

	client.BeforeSend(func(msg posthog.Message) posthog.Message {
		switch m := msg.(type) {
		case posthog.Exception:
			// would be nice if we can access the actual exception here,
			// instead of just the already transformed data
			return m
		default:
			return m
		}
	})
	client.BeforeSend(func(msg posthog.Message) posthog.Message {
		// is not sent if `nil` is returned
		return nil
	})
	client.BeforeSend(func(msg posthog.Message) posthog.Message {
		// never runs
	})

[hyeomans]

Hey @orian 👋

I took a shot at implementing the BeforeSend hook: #154

Implemented the feature with full Ruby SDK parity, comprehensive tests, and detailed documentation. Would love your feedback when you get a chance!