[FIX] product: dynamic variant creation

The new product configurator introduces the possibility to create
variants dynamically. One of the main goal is to be able to create
product templates with **many** attributes and values, but not generate
all variants automatically. Only the necessary variants are created
on-the-fly.

However, it turns out that **many** dynamic attributes and values still
fails. For example, create a product with 10 attributes, 10 values for
each. This allows 10^10 theoretical combinations; although the variants
are not created, the creation of the product template fails.

The reason is that the `variant_matrix` contains the list of all
combinations, which causes a `MemoryError`.

An easy modification to improve the process is to keep an iterator
instead of generating the list. This avoids the `MemoryError`, but it
remains necessary to go through all combinations three times, which
would take too much time.

Therefore, some refactoring is necessary in order to keep the same
logic, but avoid going through any huge list. Here are the improvements:

1. Skip the creation part if nothing will be created.
   If any attribute is dynamic, no variant is created. Therefore, we can
   perform the check upfront and skip the variant creation part if
   possible.

2. Stop the creation loop as soon as possible.
   There is an hardcoded limit of maximum 1000 variants to create. We
   can perform this check along with the list creation instead of
   checking at the end.

3. Determine if a product has valid attributes in a different way.
   The existing logic is safe but expensive: we check that all attribute
   values of a product are part of the possible combinations. This can
   be simplified by checking if (1) all attribute values of a variant
   are part of the accepted attribute values of the template and (2) a
   variant uses all attributes of the template. The second check is
   mandatory to delete existing variants when an attribute is added to
   the template.

Thanks to these modifications, we at worst go through 1000 possible
attribute values combinations. It might still be possible to optimize
step 3 with a fancy SQL query which would retrieve all (in)valid
products at once. Hopefully this won't be necessary and the optimized
logic will be enough.

opw-1902392

closes odoo/odoo#28530

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