Implement fmt_as for ShuffleReaderExec

[andygrove]

Which issue does this PR close?

Before:

SortExec: [revenue DESC]
  ProjectionExec: expr=[n_name, SUM(l_extendedprice Multiply Int64(1) Minus l_discount) as revenue]
    HashAggregateExec: mode=Final, gby=[n_name], aggr=[SUM(l_extendedprice Multiply Int64(1) Minus l_discount)]
      ExecutionPlan(PlaceHolder)

After:

SortExec: [revenue DESC]
  ProjectionExec: expr=[n_name, SUM(l_extendedprice Multiply Int64(1) Minus l_discount) as revenue]
    HashAggregateExec: mode=Final, gby=[n_name], aggr=[SUM(l_extendedprice Multiply Int64(1) Minus l_discount)]
      ShuffleReaderExec: partition_locations=[executor=cb5f9a3b-69ed-4261-b3b5-38ba74ba7e30 part=50ELpmq:1:0 stats=PartitionStats { num_rows: None, num_batches: None, num_bytes: None }]

Closes #399.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #400 (030d342) into master (174226c) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
- Coverage   74.94%   74.90%   -0.05%     
==========================================
  Files         146      146              
  Lines       24314    24328      +14     
==========================================
  Hits        18223    18223              
- Misses       6091     6105      +14     

Impacted Files	Coverage Δ
...ta/rust/core/src/execution_plans/shuffle_reader.rs	0.00% <0.00%> (ø)

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