The field prefetching mechanism was poorly customizable.  Before this,
we could only tell if a field was prefetched with other fields or not at
all.  We have no way to inform the framework, like: "When I need data of
that field, prefetch these other fields, which are likely be used in the
same transaction".

From now on, the `prefetch` attribute is used as a grouping key for
prefetching fields.  When a field is fetched, all the fields with the
same value for `prefetch` are taken for prefetching.

For example, consider a small set of fields that are rarely used, except
for one flow A using them.  You want to prefetch those fields only in
the flow A, and you want to fetch them in a single query.  With the new
feature, simply set `prefetch=A` for some string `A` on those fields,
and they will be grouped for prefetching.

closes odoo/odoo#85220

Signed-off-by: Rémy Voet <ryv@odoo.com>