Description of the issue/feature this PR addresses:
This commit is a performance fix to improve the speed of opening the pos kiosk or QR code.

`_get_attributes` method of pos_self_order's product.product extension will call `self.env["pos.session"]._get_attributes_by_ptal_id()` every time it is called. For *N* products, `_get_attributes` is called *N* times. This can lead to slow performance for high enough *N*, because `_get_attributes_by_ptal_id` method is slow, because it makes many `read` calls to product.attribute.value

This commit lifts the call to `_get_attributes_by_ptal_id` higher in the call stack, so that it is only called once as opposed to *N* times. It passes its result into `_get_attributes` via context, which will re-call `_get_attributes_by_ptal_id` if it wasn't in context for backwards compatibility reasons.

attributes must be deep copied within `_get_attributes`, because `_add_price_info_to_attributes` mutates the values within, which would invalidate future calls. The deep copy gives a fresh instance for each call, and only copies the applicable attributes so it shouldn't be large.

In this particular customer's DB they have 1376 product.product records and their pos config's pricelist (id 36) has 1572 rules in it.

Overall, based on the benchmarks below, this commit makes loading the pos about 4-5 times faster.

Benchmarks:

__Before commit__

_Customer DB_
product.product count == 1376
SQL query count ~= 5034
Time to load pos ~= 44 sec

_Customer DB with more products_
product.product count == 3792
SQL query count ~= 9359
Time to load pos ~= 96 sec

__After commit__

_Customer DB_
product.product count == 1376
SQL query count ~= 2360
Time to load pos ~= 8 sec

_Customer DB with more products_
product.product count == 3792
SQL query count ~= 3906
Time to load pos ~= 22 sec


Current behavior before PR:
Slow loading of pos kiosk/QR code

Desired behavior after PR is merged:
Faster loading of pos kiosk/QR code

opw-3758923

Forward-Port-Of: odoo/odoo#159551