Implement planner for converting multiple SQL statements to unified RelNode Tree

This can be implemented as a separate planner or in VolcanoPlanner itself. The planner should take multiple SQL statements as input and return a unified RelNode tree.

Example of above is as follows:

SELECT COL1, COL2 FROM TABLE WHERE COL3 > 10;
SELECT COL1, COL2 FROM TABLE WHERE COL4 = 'abc';

The above 2 statements have a common path and hence can provide a unified RelNode tree as follows:

 [Scan] -> [Project (COL1, COL2)] -> [Filter (COL4 = 'abc')] -> [Delta]
                    |
                    V
            [Filter (COL3 > 10)]
                    |
                    v
                 [Delta]