introduce CLOSESPIDER_TIMEOUT_NO_ITEM in CloseSpider

[BurnzZ]

Motivation: Sometimes spiders still keep running without producing any items. This could be due to a variety of reasons like poor dupe URL filtration, bad crawling strategy, etc.

Proposal: Auto-close the spider if it hasn't produced any items in the past number of seconds.

[codecov]

Codecov Report

Merging #5979 (a6ccbc3) into master (8055a94) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head a6ccbc3 differs from pull request most recent head 368ab29. Consider uploading reports for the commit 368ab29 to get more accurate results

@@            Coverage Diff             @@
##           master    #5979      +/-   ##
==========================================
- Coverage   89.19%   89.15%   -0.04%     
==========================================
  Files         162      162              
  Lines       11287    11309      +22     
  Branches     1833     1836       +3     
==========================================
+ Hits        10067    10083      +16     
- Misses        928      933       +5     
- Partials      292      293       +1     

Impacted Files	Coverage Δ
scrapy/extensions/closespider.py	96.82% <100.00%> (+1.70%)	⬆️

... and 2 files with indirect coverage changes

[BurnzZ]

I initially thought of exposing this but I think it's better to keep it simple for now. Users can opt to override this method anyway if they need a customized condition of when to close the spider.

[kmike]

Looks good to me! If needed, we can add the "items in period" setting later.