[idea] Add some non-standard but commonly used HTTP status codes for retrying

[starrify]

Cloudflare is being used widely, which has has introduced its non-standard HTTP status codes. E.g.

• Error 522: Connection timed out
• Error 524: A timeout occurred

In practice, I've seen several occurrences of 522 and/or 524 status codes generated by Cloudflare. Usually a user may simply override setting RETRY_HTTP_CODES to include these values.
Here I suppose it might be good to include these two codes into scrapy.settings.default_settings.RETRY_HTTP_CODES.

See also:

• List of Croudflare error codes
• List of HTTP status codes (Wikipedia) -> Unofficial codes -> Cloudflare

[redapple]

Sounds fine @starrify

[dangra]

fixed by #2852