[bitnami/gitea] Add new customizable environment variables - OAUTH2_CLIENT

[ikamii]

[bitnami/gitea] Add new customizable environment variables

                [outh2_client]
                1. ENABLE_AUTO_REGISTRATION (default: false)
                2. USERNAME (default: nickname)

Description of the change

I have made changes in scripts gitea-env.sh, libgitea.sh and in `app.ini.template to add two new customizable environment variables to configure oauth2_client.

README.md has been updated with these changes.

Benefits

Users can enable Auto Registration while logging in via OIDC and change username claim as well

Possible drawbacks

Applicable issues

There are no open issues for these changes.

Additional information

I would also make changes in bitnami/charts/gitea to enable same features for the helm chart.