[Flagsmith] FlagsmithProvider does not implement shutdown logic

[paulhobbel]

We've recently ran into an issue where one of our applications wouldn't gracefully shutdown. After debugging we found out that the OkHttp thread created by the FlagsmithClient is not closed properly when using the FlagsmithProvider of OpenFeature.

According to Flagsmith's documentation you can call the close function to clean up all resources. It seems like the FlagsmithProvider does not implement the shutdown function and thus does not call close on the FlagsmithClient.