[14.0][IMP] shopinvader: add possibility to use a specific exporter

[acsonefho]

Add possibility to use a specific exporter for shopinvader.variant json used as response in shopinvader_request case.

By default, the M2O field is empty and keep the previous behaviour.

[simahawk]

I did something similar here https://github.com/shopinvader/odoo-shopinvader/pull/1278. Can you have a look?

[simahawk]

The build is broken w/ the same error on plain 14.0 https://github.com/shopinvader/odoo-shopinvader/actions/runs/3051196438/jobs/4919136604#step:8:212

See #1320

[sebastienbeau]

@Cedric-Pigeon @lmignon if it's ok for you, you can merge it

[lmignon]

@sebastienbeau If we merge this one it will collide with #1278 which is older.... The same field variant_exporter_idwill be used to allows to have different exporters by backends on one side and to have a custom exporter when the product's information are converted into json to be put into the cart / sale info.... For sure, it's always a bad idea to serve the same information from different sources (search engine and odoo)..... and it cost always a lot.

@acsonefho can you find a way to avoid conflicts with the pr of @simahawk