planner: support a hint or var like `index-join-first` to indicate the planner to prefer `IndexJoin` over other joins

## Enhancement
In some typical TP scenarios, for multi-way joins, the best plan should always use `IndexJoin` or `Apply`(`NestedLoopJoin`), but our optimizer sometimes might choose other join types due to inaccurate cost model or cardinality estimation.

We've met this problems multiple times, so it might be better to support a hint or variable like `index-join-first` to indicate the optimizer always choose `IndexJoin` if possible in such scenarios: