Move pos related crates into a separate folder

[Pana]

The code related to POS is currently located mainly in the crates/cfxcore/core/src/pos directory, which includes code for interacting with POW as well as many standalone crates. It is suggested to move the standalone crates to a separate directory, such as crates/pos, to avoid deep nesting in the code structure and achieve a flatter crate organization.

This will also prepare for the future split of POS into an independent consensus client.