Integrate with duckdb's explain system

[Xuanwo]

We need to output lance's own plan in duckdb like:

ProjectionExec: expr=[chunk_index@3 as chunk_index, title@4 as title, identifier@2 as identifier, _distance@0 as _distance]
  RemoteTake: columns="_distance, _rowid, identifier, chunk_index, title"
    CoalesceBatchesExec: target_batch_size=1024
      GlobalLimitExec: skip=0, fetch=100
        SortExec: TopK(fetch=100), expr=[_distance@0 ASC NULLS LAST], preserve_partitioning=[false]
          ANNSubIndex: name=vector_idx, k=100, deltas=1
            ANNIvfPartition: uuid=83916fd5-fc45-4977-bad9-1f0737539bb9, nprobes=20, deltas=1
            ScalarIndexQuery: query=AND(identifier > 0,identifier < 1000000)

[Xuanwo]

Supported in #43