Index children SKUs for configurable

[real34]

In order to match ElasticSuite for M2 format, configurable products should have a list of their own SKU + their children's SKU.

It allows to find a product by SKU of another one not visible individually.

Current

Nothing

Expected

"type_id": "configurable",
"sku": [
"VT08"
,
"VT08-RN-XS"
,
"VT08-RN-S"
,
"VT08-RN-M"
,
"VT08-RN-L"
,
"VT08-MT-XS"
,
"VT08-MT-S"
,
"VT08-MT-M"
,
"VT08-MT-L"
,
"VT08-LL-XS"
,
"VT08-LL-S"
,
"VT08-LL-M"
,
"VT08-LL-L"
,
"VT08-LA-XS"
,
"VT08-LA-S"
,
"VT08-LA-M"
,
"VT08-LA-L"
],