Pass info from FEEDS to ItemExporter

[StasDeep]

Summary

The new FeedExporter class doesn't allow to pass custom info to ItemExporter through the FEEDS setting.

Motivation

My use-case is a bit too long to describe, but it can be minimized to the following: for example, I have a custom ItemExporter which I want to have different options for different feeds. Say I want to pass custom param option to it.

class CustomItemExporter(BaseItemExporter):
    
    def _configure(self, options, dont_fail=False):
        super()._configure(options, dont_fail)
        self.param = options.pop('param', None)
        
    def export_item(self, item):
        # Using self.param here
        ...

Ideally, I would like to do set it like this:

FEEDS = {
    uri_detail: {'format': 'custom', 'item_export_kwargs': {'param' : 'Detail'}},
    uri_overview: {'format': 'custom', 'item_export_kwargs': {'param' : 'Overview'} }
}

Describe alternatives you've considered

The only workaround I've found was to create a new FeedExporter class and override open_spider method to pass feed['item_export_kwargs'] to _get_exporter method.

Additional context

Will provide a PR this week.

[Gallaecio]

I implemented this in #4512, as I needed it there.

[StasDeep]

@Gallaecio if I understand the changes correctly, it is now possible to pass it to FeedStorage. What I want, though, is passing custom args to ItemExporters.