101 featch available variables from previous steps (#8062)

- add outputSchema in workflow step settings
- use outputSchemas to compute step available variables


https://github.com/user-attachments/assets/6b851d8e-625c-49ff-b29c-074cd86cbfee

packages/twenty-front/src/modules/workflow/graphql/mutations/computeStepOutputSchema.ts

@@ -0,0 +1,7 @@
+import { gql } from '@apollo/client';
+
+export const COMPUTE_STEP_OUTPUT_SCHEMA = gql`
+  mutation ComputeStepOutputSchema($input: ComputeStepOutputSchemaInput!) {
+    computeStepOutputSchema(input: $input)
+  }
+`;