User ID gets ignored

## Expectation
If you set User ID in the MC parameters, it should get sent, and be observable in GA4 BigQuery exports.

## Observations
If you set `User ID` in the config, it gets sent with server side requests (as `uid`).

It's a little hard to tell if this works, because it doesn't show up in the DebugView in GA4. However, if you GA4 exporting to BigQuery, there's a `user_id` column for events. I found this was always `null` despite getting sent to GA4.

I have been troubleshooting by also firing requests client side with `gtag`, but was unable to figure out why the `uid` from the request wasn't showing up. The client side requests looks a bit similar, but is sent to another domain, and has many different parameters in addition to overlapping ones. Notably, the client side request with `user_id` set in the JavaScript, did map to a `uid` URL param also.

For the GA4 MC, I tried passing `G111` for consent manually, setting custom params `uid` and `user_id` manually (not what we want since it sets event params).

For events sent client side, I was able to observe in BigQuery in seconds, which also confirms there's no config issue related to my tracker or property.

## Potential solution
It seems like a lot of the current implementation is reverse engineered and depends unpublished API details to mimic a client side tag. However, there's a stable and published API for server side events. #17 (but for all events, except audiences), could solve this.

No files or tests are named. Start by tracing how the MC User ID becomes the server-side `uid`, then read issue #17 and compare the current implementation with the published server-side events API. Done means configured User ID values appear as `user_id` in GA4 BigQuery exports for supported events.

Written by the indexing model from the issue text.