Design the interface of join enumeration algorithm

The core feature of cost-based optimizer is cost-based join enumeration approach. The main objective of this is to find the best join order from lots of enumerated join trees. In database community, there have been many studies for join enumeration ways.

It would like to great to provide a generic and pluggable interface for join enumeration algorithms. It may encourage contributors to implement various join enumeration ways, and users could choose the best join ordering algorithm for their purpose.