Incomplete proxy support

[3hhh]

This plugin only supports proxies directly via splash [1].

Using the native scrapy proxy support [2] leads to an attempt to contact splash via the configured proxy.

It would be more optimal to transparently pass the scrapy proxy arguments to splash as it's currently done with headers & cookies. Currently users have to write their own middleware for that.

[1] https://splash.readthedocs.io/en/stable/scripting-request-object.html?highlight=proxy#request-set-proxy
[2] https://docs.scrapy.org/en/latest/topics/downloader-middleware.html#scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware