attempt to discover preferred proxy in extended profile before using default

[elf-pavlik]

extended profile can include preference for proxy service and client should attempt to discover it first before falling back to use some default proxy service.

[dmitrizagidulin]

This is a good idea. I think it belongs more in https://github.com/solid/solid-spec rather than here in the client lib. (although proxying is a client concern, specifying how to discover it from a profile is a general spec concern.)

[elf-pavlik]

I'll make PR to solid/solid with proposal for proxy discovery.
Still solid-client will need to implement it and if discovered in extended profile, then configure solid-web-client to use it.