Cleanup: Move geometry set fields to a separate header

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

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