[ebay Search API] Support flexible filter options

[hilmanski]

A user is interested in filtering a search result by "region code" and "platform." Upon investigating this on eBay's side, it looks like eBay let these be individual parameters on the URL. I'm afraid it'll be too much work if we support the filters individually since each search will show different filter options.

I'm not sure if this is visible, but is it possible to have a "flexible" parameter that supports any kind of filter on eBay? One pattern that I noticed for these "flexible parameters" is that they use capital letters at the beginning.

another example: https://github.com/serpapi/public-roadmap/issues/1651

[Update] One idea is introducing the extra_filters parameter as a JSON parameter similar to the multi_city json we have on Google Flights API https://serpapi.com/google-flights-api#api-parameters-advanced-google-flights-parameters-multi-city-json

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