Rename SemiJoin's JoinerParams.probe_is_join_side to is_probe_on_outer_side

[xumingming]

I was reading the semi join implementation and find that the field name: JoinerParams.probe_is_join_side is hard to understand its meaning:

auron/native-engine/datafusion-ext-plans/src/joins/bhj/semi_join.rs

Lines 60 to 64 in d357cbc

	pub struct JoinerParams {
	probe_side: ProbeSide,
	probe_is_join_side: bool,
	mode: SemiMode,
	}

join_side is NOT an adjective, it is a noun, which can be LEFT or RIGHT

auron/native-engine/auron-serde/proto/auron.proto

Lines 678 to 681 in d357cbc

	enum JoinSide {
	LEFT_SIDE = 0;
	RIGHT_SIDE = 1;
	}

After some digging I think what probe_is_join_side want to say is: whether the probe side is the same as the outer side of the join. e.g. For a left join, if the probe side is on the left, then probe_is_join_side is true, otherwise false. BTW: For a left join, left side is the outer side, right side is the inner side.

So I suggest to rename probe_is_join_side to is_probe_on_outer_side and add some comments to make it more easier to understand.