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.