321114b Locks the mutex in Diagnostics to prevent threading issues around setting the current StatsigUser

Some crashes were reported around PostWithRetry calling back to references that no longer exist.

• a059fdb chore: use shared_ptr for EventLogger

• Moving to std::shared_ptr for all callback functions to prevent capturing this.
• Being far more defensive around references

"204" refers to the status code that is returned from a /initialize call when there is not change from the Statsig server.
When this status code is returned, the SDK considers its cache values up-to-date and does not need new values from a remote server.

A bug was found where 204 was being returned in cases where the StatsigUser object changed and was no longer valid for its cached evaluations.

Remediations:

• 204 was turned off for all previous versions
• 204 will now only work in this version and above
• A check was added to only request 204 when the user object is 100% identical to that which made the original cache entry

Contains internal changes to better support the Statsig Unreal plugin.

Full Changelog: 0.0.5...0.0.6

• Uses std::filesystem::path for cache paths
• Adds StatsigOptions.cache_directory, allowing custom cache location
• Adds Layer::GetGroupName()
• Adds Experiment::GetGroupName()

• Adds StatsigOptions.environment 3fbc258
• Adds a timeout to StatigClient.Shutdown() 8c2f053

• Adds compatibility for the Statsig Unreal plugin (https://github.com/statsig-io/unreal-plugin)
• Adds StatsigResultCode to Initailize and UpdateUser functions
• Adds support for Synchornous (Cahce) and Asynchornous (Network) initialize