[16.0][ADD] product_attribute_value_group: Predefined groups of attribute values

[gabonog]

Is your feature request related to a problem?
Yes. When configuring product templates with attributes that have a large number of possible values (e.g., shoe sizes, colors, technical specifications), users must manually select each value every time they use the attribute.

This process is:

• Time-consuming: For attributes like shoe sizes, a user might need to select 10-15 values for each product.
• Error-prone: It's easy to forget a value or select an incorrect one, leading to inconsistent variant configurations across the product catalog.
• Difficult to maintain: If a standard set of values changes (e.g., a new size is added to the "EU Men's Sizes" range), there is no centralized way to update all affected products.

The existing OCA module product_attribute_set helps by grouping attributes, but it doesn't solve the problem of selecting the values within each attribute line.

Describe the solution you'd like
I propose creating a new module, product_attribute_value_group, that allows users to define and reuse predefined sets of attribute values.

Describe alternatives you've considered
Extending product.attribute directly: We could have added the sets directly to the attribute model, but a separate model (product.attribute.value.group) is cleaner, more scalable (an attribute can have multiple sets, like "Men's Sizes" and "Kid's Sizes"), and follows Odoo's design patterns for configurable data.

Additional context
This feature would be highly beneficial for industries like fashion, footwear, and electronics, where standardized sets of options are common. It complements the existing product_attribute_set module by addressing the next logical step in the configuration process.