Provide control over setting client_id and user_id in common event

[adatzer]

At the moment the client maps the uid/user_id Snowplow property to the user_id property of the common event (here and here). Sometimes this may not be desirable (for example when using the Snowplow Client in combination with GA4 Tag).

Even though gtm-server-side tags usually provide options to configure any mapping downstream, we could also explore providing an option to disable this mapping from the Client, while also leaving the original behaviour as default.

Edit: We could provide a common pattern to also take into account #26