Initial commit: Extract shared plugins from hive

Plugins extracted:
- json_rpc: JSON-RPC 2.0 API framework
- webserver: HTTP/WebSocket server plugin
- statsd: StatsD metrics collection
- app_status_api: Application status API

These plugins are blockchain-agnostic and can be used by:
- hive (with HIVE_BUILD_WITH_BLOCKCHAIN=ON)
- beekeeper (standalone wallet daemon)

Dependencies (as submodules):
- fc (hive/fc)
- appbase (hive/appbase)