[Feature Request]: Add a Variant type to Beam Schemas

[ahmedabu98]

What would you like to happen?

PCollections of Beam Rows have fixed schemas, making it hard to process records with different logical schemas.

We need a semi-structured type that can contain a nested level of variable columns. Transforms can choose to unwrap this type to reconstruct the variable columns accordingly.

The Variant type is a becoming a good standard across different projects (see Parquet](https://parquet.apache.org/docs/file-format/types/variantencoding/), Spark, Flink). It could also be a good candidate for Beam.

Bonus if such a type is made portable so that other SDKs can use it too

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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