Add support for forwarding proxy

[TolstochenkoDaniil]

Prefase 📝

I am using scrapyd-client utility to deploy spiders on prod server.
In case to avoid my spiders to be banned, IT support configured forward proxy to hide host IP.

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Issue ⚠️

After forward proxy was enabled, scrapy-deploy target -p project command was not available any more.
Server responded with 503 - Forwarding failure (Privoxy@unknown) error and such message:

Privoxy was unable to socks5-forward your request http://xxx.xxx.xxx.xxx:6800/addversion.json through 127.0.0.1: SOCKS5 request failed

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Work around 🛠️

I was able to find solution to deploy spiders to JSONAPI scrapyd server.

1. Create .egg file

run scrapyd
execute setup.py python setup.py bdist_egg

2. Make post request as follow

curl --noproxy '*' http://<your_host>:/addversion.json -F project=<project_name> -F version=<version_name> -F egg=@<path_to_egg>

After doing steps i've mentioned above, i finally managed to deploy project. ✅

Hope, this would help someone in future! ✌🏼