OpenID Connect client auth flow

[rvolosatovs]

The client crate should provide functionality to simplify the login flow where possible.

CLI should contain the minimum complexity required and handle all interactive functionality, as well as printing to stdout/stderr.

Essentially, we should make it simple to acquire a token for the CLI by using just the functionality in this crate. How exactly to do that is up to the implementor to decide.

See example at https://docs.rs/openidconnect/2.3.1/openidconnect/index.html#getting-started-authorization-code-grant-w-pkce.
Note, that the implementation should https://docs.rs/openidconnect/2.3.1/openidconnect/struct.ProviderMetadata.html#method.discover on the issuer URL

[rvolosatovs]

Refs #194