security/openvpn-auth-oauth2: Add new port

openvpn-auth-oauth2 handles the single sign-on (SSO) authentication
for OpenVPN servers. Authentication can be performed against
various identity providers, among others also Microsoft Entra ID,
GitHub, Okta, Google, Keycloak and other OIDC-compliant providers.

Docs are at https://github.com/jkroepke/openvpn-auth-oauth2