[OAuth OIDC] Qt OAuth2 for OpenIDConnect id_token acquisition

OpenIDConnect (OIDC) is a flow used for authenticating users. This is distinct from authorization; the goal of authorization is to give a permission to do something, where as with authentication the goal is to identify the user reliably.

OIDC is a light layer on top of OAuth2 flows. As such, qtneworkauth module doesn't provide out-of-the-box support for OpenID Connect flow. But since it's a light layer on top of OAuth, OIDC flows can be implemented with reasonable effort.

The scope of this task is to make it easier to acquire the id_token. This task does not include validating the token (signature verification, and if encrypted, decryption) or other OIDC concepts such as endpoint discovery or userinfo endpoint usage. The received id_token can be used as an opaque token similar to access token, and also since the id_token (JWT) is a JSON object, reading the it's fields should be easy.