Client authentication

[tusmester]

We want to add support for a modern authentication process in the sensenet client. The easiest way to do that is to add IdentityServer4 support.

Required package reference: IdentityModel

Requirements

• authentication is server-specific, not a global setting
• no breaking change, the previous login api and auth workflow should still work (basic or NTLM auth)
• hide token refresh mechanism from callers (client developers), that should be done automatically by the client library internally
• the client library itself does not save/serialize token data and cookies, only handles in-memory info
• the caller should be able to get and set all auth data in a serializable format (so that users do not have to give their un/pw every time they start the client app)

[kavics]

I would like to see a sequence-diagram that specifies the per-request communication among the application / server / authentication provider triplet. We need to do such diagram.