[FLINK-26340][statefun-golang-sdk] Add ability in Golang SDK to create new statefun.Context from existing one, but with a new underlying context.Context

Change sync.Mutex in statefunContext to be *sync.Mutex, to make it copyable, and update construction of statefunContext accordingly
Add WithContext to statefun.Context interface and implement on statefunContext
Add unit test

This closes #303
4 files changed