Creates a new custom OIDC/OAuth provider.
For OIDC providers, the server fetches and validates the OpenID Connect discovery document from the issuer's well-known endpoint (or the provided discovery_url) at creation time. This may return a validation error (error_code: "validation_failed") if the discovery document is unreachable, not valid JSON, missing required fields, or if the issuer in the document does not match the expected issuer.
This function should only be called on a server. Never expose your service_role key in the browser.