All struct-aware optimizations are hardcoded to `GetFieldFunc`

### Is your feature request related to a problem or challenge?

Every optimization for struct field querying (row level pushdown, leaf-level projection pruning, `PushdownChecker`, `resolve_struct_field_leaves`) is gated on the `GetFieldFunc` udf.

A custom UDF like `variant_get` gets none of these optimizations. There's no trait or capability API for a UDF to declare "I'm a struct field access."

This blocks Variant support entirely unless `variant_get` decomposes to `get_field` chains or the Parquet source learns a general mechanism for struct-access UDFs

Start by tracing the GetFieldFunc gates in row-level pushdown, leaf-level projection pruning, PushdownChecker, and resolve_struct_field_leaves. Compare how the Parquet source handles struct access and define how a capability API could let custom UDFs such as variant_get receive the same optimizations; done means those paths no longer depend solely on GetFieldFunc.

Written by the indexing model from the issue text.