fix: Updated gotrue types.dart to include slackOidc (#1066)

Updated gotrue types.dart to include slackOidc

Gotrue exported OAuthProvider doesn't currently include Slack OIDC provider option. Therefore, trying to pass OAuthProvider.slack to sign in will set the wrong "provider" url param.

Author: Oskar-Nilsen-Roos

packages/gotrue/lib/src/types/types.dart

@@ -25,6 +25,7 @@ enum OAuthProvider {
   linkedinOidc,
   notion,
   slack,
+  slackOidc,
   spotify,
   twitch,
   twitter,