[C++][Parquet] Add variant support

[neilechao]

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++

[adriangb]

It would be great for folks working on this here and in other languages if some example files could be added to https://github.com/apache/parquet-testing 🙏🏻

[wgtmac]

Issue resolved by pull request 45375
#45375

[neilechao]

@wgtmac - we should probably keep this open until all of the sub-issues are done. Should I can create a sub issue for the logical type itself and attach the PR-45375 to that new sub issue?

[zeroshade]

@wgtmac we should probably consider adding variant as a canonical extension type?