Support opitonal Closeable interface for other stateful objects

[jchrys]

While Connection already implements Closeable as a mandatory part of R2DBC, other stateful objects such as ConnectionFactory could benefit from providing a way to release their resources through implementing the Closeable interface.

This is an optional feature, so it is not mandatory to implement it according to the R2DBC specification.

However, it would be useful for those who need to manually close their resources without blocking the caller.

refer: https://r2dbc.io/spec/1.0.0.RELEASE/spec/html/#closeable