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GH-14775: [Go] Fix UnionBuilder.Len implementations (#14776)

(*DenseUnionBuilder).Len and (*SparseUnionBuilder).Len always return 0. They should instead return the number of values appended.

This bug exists because these methods are promoted (via unionBuilder) from (*builder).Len, which reads builder.length, but DenseUnionBuilder and SparseUnionBuilder don't update that field.

Fix by adding a (*unionBuilder).Len implementation that calls unionBuilder.typesBuilder.Len.

Authored-by: Noah Treuhaft <noah.treuhaft@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>