Provide utility to visualize RelNode plans

From time to time, we want to visualize the plan generated the optimizer as a DAG (Directed Acyclic Graph). This gives us an overview of the plan, and helps us to find problems with the plan quickly.

A common way to visualize a DAG is to dump it in the dot format, and then display the DAG through GraphViz. Currently, we already have a utility to dump the Volcano planner in dot format. In this issue, we want to dump a RelNode plan (a RelNode DAG) in the dot format, which will help users to see their plans more clearly.

The utility should have some options that allow users to specify the visuzalization details, for example:
1. the max length of a node description.
2. whether RelSubset/HepRelVertex should be displayed.
3. Nodes that should be highlighted.

Could you please give some feedback?