Extend schema to contain table name, and migrate cost model's ColumnRef property usage to schema

[jurplel]

From Chi:

yes we should do that, feel free to create an issue...
this is so-called "column name inference"
assign a name for unnamed columns, and use user-specified names for projections
and for the top-level node, if we have physical property implemented, it will always be a gather node
unless all plan nodes are singleton

ColumnRef and Schema seem to have a lot of purpose overlap, I think that we can merge their functionality together.