Minor Description
A first step towards allowing file format implementations to inject custom metadata fields into plans is to make the handling of metadata attributes in `FileSourceStrategy` more generic.
Today in `FileSourceStrategy` , the lists of constant and generated metadata fields are created manually, checking for known generated fields on one hand and considering the remaining fields as constant metadata fields. We need instead to introduce a way of declaring metadata fields as generated or constant directly in `FileFormat` and propagate that information to `FileSourceStrategy`.