[che-operator] Enhancements in the way how OAuth2 providers can be configured

Synced from Eclipse Che issue

https://github.com/eclipse/che/issues/20496

Is your task related to a problem? Please describe

• Github, Gitlab, Bitbucket should be configured in a similar way.
• Factory OAuth flow without Keycloak.

Describe the solution you'd like

• Should be possible to configure Gitlab from secret. In the same way, like it's made for Github and Bitbucket.https://github.com/eclipse/che/issues/18932
• Github and Gitlab OAuth configs should be mounted as a file to che-server in the same way like it's made for Bitbucket
• Environemnt variables for clientid and secret. CHE_OAUTH2_GITHUB_CLIENTID__FILEPATH CHE_OAUTH2_GITHUB_CLIENTSECRET__FILEPATH
• Change path /home/user/eclipse-che/conf/oauth1/bitbucket/ - > /config/oauth1/bitbucket/ and /config/oauth2/github/ and /config/oauth2/gitlab/ see more https://github.com/eclipse/che/issues/19061#issuecomment-915169002
• Add CHE_INTEGRATION_GITLAB_SERVER__ENDPOINTS. Like https://github.com/eclipse-che/che-operator/blob/8e0966bed3c682f81bba2c2454ace33cb1e9c761/pkg/deploy/server/server_configmap.go#L314-L334

Release Notes Text

The way to configure GitHub, Bitbucket and Gitlab as Git service providers for Che workspaces has been made consistent and simpler.