[SPARK-53066][SQL] Improve EXPLAIN output for DSv2 Join pushdown

[PetarVasiljevic-DB]

What changes were proposed in this pull request?

Prior to this change, EXPLAIN FORMATTED on query e.g.:

SELECT *
FROM catalog.tbl1 t1
    JOIN catalog.tbl2 t2 ON t1.id1 = t2.id2
    JOIN catalog.tbl3 t3 ON t2.id2 = t3.id3
    JOIN catalog.tbl4 t4 ON t3.id3 = t4.id4;

looked like:

PushedJoins: [join_pushdown_catalog.tbl1, join_pushdown_catalog.tbl2, join_pushdown_catalog.tlb3, join_pushdown_catalog.tlb4]

With the change from PR, the output of EXPLAIN FORMATTED would be:

(1) Scan JDBC v1 Relation from v2 scan join_pushdown_catalog.JOIN_SCHEMA.JOIN_TABLE_1 [codegen id : 1]
Output [14]: [ID_c0e665d3_3cea_4fb8_b57d_6a9f26af360f#14, ID_03c3fe2f_6cd9_4794_ace9_85e617420548#17,  ID_ea300272_c11a_4c0c_aed8_a83a60f3da33#21, ID#24]
PushedFilters: [ID_ea300272_c11a_4c0c_aed8_a83a60f3da33 = (ID + 1)]
PushedJoins: 
[0]: [PushedFilters: [ID_03c3fe2f_6cd9_4794_ace9_85e617420548 = (ID + 1)],
    PushedJoins: [
    [0]: [PushedFilters: [ID_c0e665d3_3cea_4fb8_b57d_6a9f26af360f = (ID + 1)],
        PushedJoins: [
        [0]: [Relation: join_pushdown_catalog.tbl1, PushedFilters: [ID IS NOT NULL]],
        [1]: [Relation: join_pushdown_catalog.tbl2, PushedFilters: [ID IS NOT NULL]]
      ]],
    [1]: [Relation: join_pushdown_catalog.tbl13, PushedFilters: [ID IS NOT NULL]]
  ]],
[1]: [Relation: join_pushdown_catalog.tbl4, PushedFilters: [ID IS NOT NULL]]

PushedFilters on top of PushedJoins are actually join conditions.
It can be seen that the name of Scan JDBC v1 Relation from v2 scan is catalog.tbl1. This should be fixed as well, but it won't be a part of this PR.

Why are the changes needed?

Does this PR introduce any user-facing change?

How was this patch tested?

Was this patch authored or co-authored using generative AI tooling?

[PetarVasiljevic-DB]

cc: @cloud-fan