Describe the enhancement requested

Variant encoding spec: https://github.com/apache/parquet-format/blob/d784f11f4485e64fdeaa614e0bde125f5132093d/VariantEncoding.md

Variant shredding spec: https://github.com/apache/parquet-format/blob/d784f11f4485e64fdeaa614e0bde125f5132093d/VariantShredding.md

Loose order of steps:

1. GH-45937: [C++][Parquet] Logical type definition for variant #46104
2. GH-45937: [C++][Parquet] Variant decoding #45946
3. GH-45937: [C++][Parquet] Variant encoding #45947
4. GH-45937: [C++][Parquet] Variant shredding #45948

Component(s)

Parquet, C++