Add support for custom events - log_event

[madhuchavva]

This PR adds the ability to log custom events using log_event supported by the existing GrowthBookTrackingPlugin.

Usage

Sync client (GrowthBook)

from growthbook import GrowthBook, growthbook_tracking_plugin
gb = GrowthBook(
    client_key="sdk-abc123",
    attributes={"id": "user-42", "user_id": "u42", "utmSource": "google"},
    plugins=[
        growthbook_tracking_plugin(
            ingestor_host="https://us1.gb-ingest.com",  # default, can omit
            batch_size=20,
            batch_timeout=5.0,
        )
    ],
)
# Anywhere in your request handler:
gb.log_event("checkout_started", {"cart_value": 99, "currency": "USD"})
gb.log_event("button_clicked", {"button_id": "cta-hero"})
# Flush + teardown (e.g. app shutdown)
gb.destroy()

Async client (GrowthBookClient)

import asyncio
from growthbook import GrowthBookClient, growthbook_tracking_plugin
from growthbook.common_types import Options, UserContext
client = GrowthBookClient(
    Options(
        client_key="sdk-abc123",
        tracking_plugins=[
            growthbook_tracking_plugin(
                ingestor_host="https://us1.gb-ingest.com",
                batch_size=20,
                batch_timeout=5.0,
            )
        ],
    )
)
async def handle_request(user_id: str):
    await client.initialize()
    user_ctx = UserContext(attributes={"id": user_id, "user_id": user_id})
    await client.log_event("page_view", {"path": "/dashboard"}, user_ctx)
    await client.log_event("button_clicked", {"button_id": "upgrade-cta"}, user_ctx)
asyncio.run(handle_request("user-42"))

How it works:

• Without the plugin → log_event logs a WARNING (no-op for the user's app)
• With the plugin → events are batched and flushed via background thread, exactly as now.
• set_event_logger is a public API, just like the setEventLogger in JS SDK.