Add Dataset Deal DSL Go example

[longquanzheng]

Summary

• add a Dataset Deal DSL finite-state-machine example for seller-defined processes and independent buyer executions
• store reusable process definitions in PostgreSQL while snapshotting execution state into Dex attributes
• add REST APIs and an embedded seller/buyer UI for creating processes, starting executions, sending condition messages, and listing executions
• add Docker Compose, search-attribute registration, and a standalone end-to-end demo script

Rationale and user impact

This example demonstrates how a fixed set of Go SDK steps can interpret a seller-authored state machine without dynamic worker registration. Buyers can negotiate, purchase sample or full datasets, request refunds, and inspect execution state through Dex visibility and attributes.

PostgreSQL stores only reusable process definitions. Each execution copies its definition into Dex, so later database edits do not change a running deal.

Validation

• make -C examples/go unitTests
• make -C examples/go e2eTests
• make -C examples/go datasetDealDemo
• make copyright-check
• gofmt -d on all changed Go files
• shell syntax, JavaScript syntax, JSON, and Docker Compose validation