[REQUEST]Allow configuring OIDC parameters (e.g., discovery URL) for on-premise providers like Keycloak

[tkyggit]

Feature Request: Currently, better-auth supports public OIDC providers like Google, but it does not work with on-premise providers such as Keycloak.

The main issue is that parameters like discoveryUrl appear to be hardcoded. Please provide a way to configure these parameters via environment variables (or other configuration options) so that custom or on-premise OIDC providers can be used.