Motivation:

• Inspired by the unasync approach successfully used by projects like elasticsearch-py, This PR is the first step toward supporting asyncio clients. (related Support asyncio #2)
  To avoid maintaining two separate, parallel codebases (one for sync, one for async), this PR introduces the unasync build system. This approach allows us to write all client logic as asynchronous code (in centraldogma/_async/) and then automatically generate the synchronous version from that source.
• This initial PR only sets up the infrastructure and workflow for unasync. No actual client logic (ContentService, Dogma and so on) has been migrated yet. That will be done in follow-up PRs.

Modifications:

• Build System:
  • Adds unasync as a development dependency.
  • Adds utils/run-unasync.py as the main script to run code generation.
  • Creates the new centraldogma/_async/ directory, which will become the "source of truth" for all async/sync client code.
  • Drop transport-level retries to provide a same behavior between sync and async, and to avoid unexpected double retries.
• CI:
  • Following the elasticsearch-py strategy, this PR adds a "Check unasync" step to the workflow.
  • This CI check fails if a PR modifies _async/ code but does not also commit the corresponding auto-generated sync code. This ensures the source and generated code are always in sync.
• Updates CONTRIBUTING.md to explain this new "Code generation" workflow to all contributors. (partially moved from README.md)
• misc.
  • Fix validation when tagging
  • Rename test.yml to ci.yml
  • Move scripts/release.py to utils/run-release.py for consistency.

Result:

• BaseClient async and sync codes are maintained using a single file.
• Improved documentation to contribute.