Enable progressive proxy via flag

[jwindgassen]

With the current implementation for progressive proxies from #479, we only make the request progressive if the Accept Header of the request is exactly text/event-stream. We would like to use this feature with other proxies, where the Mime-Type is not so directly determinable, like with xpra (Remote Destop).
As far as I can tell, there is no particular reason to limit the progressive feature to only this use case (see #502).

I changed the code to instead allow configuring the behavior of the proxy via the entrypoint of a proxy. The dictionary returned by the entry point can now contain a value "progressive". When set to either True or False, we can explicitly enable or disable the progressive proxying. When left empty, we will automatically determine what type of proxy to use (see the progressive property).
It can also be a function, when the developer wants to dynamically switch between progressive and buffered proxying.

[jhgoebbert]

Great. This might also fix the issue described in #494