Add steps to AI catalog flow create service, mutation and type

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How to set up and validate locally

    # Create agent
    

    # Create flow
    mutation f {
      aiCatalogFlowCreate(
        input: {
          projectId: "gid://gitlab/Project/1000000"
          name: "Flow"
          description: "Best flow ever"
          public: true,
          steps: [
            {
              agentId: "gid://gitlab/Ai::Catalog::Item/51"
            }
          ]
        }
      ) {
        item {
          id
          name
          description
          public
					latestVersion {
            ...on AiCatalogFlowVersion {
              steps {
                nodes {
                  agent {
                    id
                    name
                    description
                  }
                }
              }
            }
          }
        }
        errors
      }
    }

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Related to #559157 (closed)