feat: add INSECURE oidc email fallback flag for IdP brokers (#26751)

<!-- Authored by Coder Agents on behalf of @Emyrk. -->

Adds an opt-in `CODER_DANGEROUS_OIDC_EMAIL_FALLBACK` flag (alias
`--dangerous-oidc-email-fallback`) for IdP brokers that do not issue a
stable `sub` for the same user across connections.

cli/server.go

@@ -280,6 +280,7 @@ func createOIDCConfig(ctx context.Context, logger slog.Logger, vals *codersdk.De
 		IconURL:             vals.OIDC.IconURL.String(),
 		IgnoreEmailVerified: vals.OIDC.IgnoreEmailVerified.Value(),
 		PKCEMethods:         pkceSupport.CodeChallengeMethodsSupported,
+		EmailFallback:       vals.OIDC.EmailFallback.Value(),
 	}, nil
 }