Statsig

Now the SDK only requires either a userID or any customID

Previously exposures for Layers happen when you call `getLayer()`, but when you have multiple experiments running in parallel in the same Layer, this could be exposing more users than the actual number of users who were actually exposed to some of the experiments, so with this release we have moved the exposure logic to be happening when `Layer.get()` method is called, so that a user is only exposed to an experiment when the parameter for that experiment is fetched.

Add debug logging information, stringify user object more gracefully, add server session id header, copy user on normalization rather than modifying

- added a new API `get_layer`, which returns a layer object that would return the correct parameter value for the given user within the layer, no matter which experiment the user is allocated to within the layer;
- fixed a bug where `time` was not added to events;
- fixed a bug where `/get_config` fallback was not correctly called in the event that the SDK does not know how to evaluate a rule locally.

Improved the ID list sync'ing logic so that it's using a new endpoint and can scale better with larger lists

Added two parameters to `StatsigOptions`:

`bootstrap_values: str = null`

a string that represents all rules for all feature gates, dynamic configs and experiments. It can be provided to bootstrap the Statsig server SDK at initialization in case your server runs into network issue or Statsig server is down temporarily.

`rules_updated_callback: typing.Callable = None,`

a callback function that's called whenever we have an update for the rules; it's called with a logical timestamp and a JSON string (used as is for bootstrapValues mentioned above). Note that as of right now, this will be called from a background thread that the SDK uses to update config values.