[Enh]: nw.Expr.struct.unnest

[benrutter]

We would like to learn about your use case. For example, if this feature is needed to adopt Narwhals in an open source project, could you please enter the link to it below?

Support unnest functionality for struct columns. Should unpack elements of struct column into individual columns.

Please describe the purpose of the new feature or describe the problem to solve.

Designed to make working with complex struct data easier (probably most likely to happen in case of json ingestion)

Suggest a solution if possible.

Implementation of function using native functions for dataframes.

If you have tried alternatives, please describe them below.

No response

Additional information that may help us understand your needs.

Useful info / links in case anyone is looking to pick this up:

• Polars docs
• DuckDB unnest docs
• I can't find specific spark docs for it, but stack overflow article mentioning the feature is here
• Ibis has 'unpack' (also a function called unnest but that's for arrays)