[FEA] Support Sort on nested struct

[viadea]

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Support Sort on nested struct.

Currently the Sort can only work on nested struct:

!Exec <SortExec> cannot run on GPU because not all expressions can be replaced
  !Expression <SortOrder> col#74 ASC NULLS FIRST cannot run on GPU because expression AttributeReference col#74 produces an unsupported type StructType(StructField(name,StructType(StructField(firstname,StringType,true)...

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GpuSort should work on nested struct.

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[viadea]

This use case is to sort on a single-level struct or a two-level struct with primitive type.

[sameerz]

For this use case we will only support the default null ordering in Spark, and document that limitation.