Implement search support

[Gallaecio]

To do:

• Improve interoperability with form2request and Scrapy Cloud support scrapinghub/Formasaurus#3
• Add a minimal action I/O implementation to improve SearchAction support scrapinghub/extruct#237
• keyword → query zyte-common-items#112
• Add BaseSearchRequestTemplatePage, change SearchRequestTemplatePage zyte-common-items#113
• Log an error when a search request cannot be built from a start URL.

[Gallaecio]

I was planning to log an error when we cannot build a request, but it was my bad to think we did not because no error was logged in the spider, we log it from the page object already and have tests for it. 🤦