feat(SDK-2185): FDv2 streaming base, initializer, and synchronizer#267

kinyoklion wants to merge 4 commits intomainlaunchdarkly/flutter-client-sdk:mainfrom

rlamb/sdk-2185/fdv2-streaminglaunchdarkly/flutter-client-sdk:rlamb/sdk-2185/fdv2-streamingCopy head branch name to clipboard