- Add Netty transport client implementation and default cluster token client
- Add config manager for cluster client
- Add codec SPI mechanism for client entity decoder / writer

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Add a `ClusterTransportClient` for transport abstraction of Sentinel cluster
- Add universal request/response interface and common ClusterRequest/ClusterResponse abstraction
- Add common request/response data entity
- Add basic abstraction of codec (EntityWriter and EntityDecoder)

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>