v0.6.0 - Adds bootstrap_values and rules_updated_callback

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.