feat(planner_v2): support standalone VALUES clause

[wangrunji0408]

This PR adds support for standalone VALUES clause (without INSERT).

> \v2
switched to planner v2
in 0.000s
> values (1, 'gg'), (0.1, true);
+-----+------+
| 1   | gg   |
| 0.1 | true |
+-----+------+

Previously the VALUES clause must be used within an INSERT statement, so we can infer the output types of the VALUES plan. But this will no longer work for a standalone VALUES. To solve this problem, we extend type analysis to plan nodes and introduce the STRUCT(..) type to describe the output types for plans. We also define the union of 2 types, which is their minimum compatible type. Then the type of a VALUES plan can be inferred as the union of all types of its rows.

This PR also fixes some bugs in the new binder, and completes EXPLAIN for all plans.