Make embedded OAuth authentication model default for both singe and multiuser che

[skabashnyuk]

Is your task related to a problem? Please describe.

At this moment there are two models of how OAuthAPI is made in Eclipse Che.

1. EmbeddedOAuthAPI - used in single-user che. In this case, we have the ability to configure GitHubOAuthAuthenticator with clientID and clientSecret to be able to get github tokens from OAuthAuthenticationService
2. DelegatedOAuthAPI - use in multy-user Che. In this mode, all OAuth activities delegated to Keycloak that is configured with Che. To be able to get an OAuth token user has to provide a valid keycloak token.

Now with Che-Theia we have an issue #14217 . It happens because we have no mechanism to convert machine-token to keycloak

Describe the solution you'd like

1. Set Embedded mode by default for single and multi-user che.
2. Provide an ability to configure Delegated mode.
3. If Embedded is set up and github's clientID and clientSecret are set for che-operator then correctly configure GitHubOAuthAuthenticator

Describe alternatives you've considered

n/a

Additional context

n/a