Upstream OAuth 2.0 providers: Support signed userinfo and customising the expected id_token signature algorithm

[MatMaul]

Add options to upstream OAuth config to specify the expected signing algorithm for the endpoint JWT responses.

[sandhose]

Thanks for this!

A few things to fix:

• id_token_signed_response_alg shouldn't be optional, I haven't seen that specced anywhere
• userinfo_signed_response_alg should default to None, as it's the most common case

[sandhose]

Almost there, just still would like to have the userinfo_signed_response_alg default to Option::None

[sandhose]

LGTM, thanks a lot!