This commit moves declarations that depend on `FN_field.hh` out of `BKE_geometry_set.hh` into `BKE_geometry_fields.hh`. This helps to reduce the number of areas that need to depend on the functions module, which recently came in in review of D11591. In the future we may have a library of standard field inputs in order to make composing algorithms easier, so it makes sense to have a header that could contain them and some basic related utilities relating the concepts of geometry and fields. Reducing use of unnecessary headers may also reduce compilation time. Differential Revision: https://developer.blender.org/D14517