Allow struct field access projections to be pushed down into scans

[adriangb]

Closes #19387

The idea is to enhance the current heuristic rules of what expressions get pushed down passed which operators to allow expressions themselves to declare that they are "trivial" (using pre-existing terminology), which currently is just columns and literals, so that struct field access (and in the future variant field access) can declare themselves as trivial expressions.

TODO:

• add tests, using a custom DataSource (Parquet doesn't actually allow pushdown of expressions referencing struct columns, to be tackled separately)
• add good PR description, improve docstrings
• Link to Redesign projection pushdown to be more granular #19550