Add support for testing different Dynamic Configs, add shutdown tests

[yuandrew]

What was changed

Added support for testing different Dynamic Config settings via existing .config.json, and added tests for frontend.enableCancelWorkerPollsOnShutdown and shutdown.

From .config.json, you can specify both the dynamic configs to set, as well as expectNamespaceCapabilities to assert that the dynamic config was actually set for each run.

• Added runVariants support to the runner.
• Each variant runs in its own embedded dev server, namespace, and task queue.
• Variant dynamicConfig values override the repo base dynamic config from dockerfiles/dynamicconfig/docker.yaml.
• Added expectNamespaceCapabilities validation via DescribeNamespace before each variant runs.
• Exposed validated capability expectations to feature code through FEATURE_NAMESPACE_CAPABILITIES for variant-
  specific assertions.

The flow I'd expect for a new feature:

1. Write feature, merge to main
2. Write features tests, then run locally against local repo (instructions in base README)

• with something like go run . run --lang go --version ../sdk-go --no-history-check worker_shutdown/poll_complete_on_shutdown

3. If regressions found, have opportunity to merge a fix in before cutting a release
4. Cut release
5. Merge features tests
6. All subsequent PRs will have new and legacy test coverage.

Why?

More test coverage for legacy behavior

Checklist

1. Closes Shutdown Worker features tests, enableCancelWorkerPollsOnShutdown #835

2. How was this tested:
   Added a bunch of tests

3. Any docs updates needed?

[Sushisource]

Overall seems perfectly sensible to me

My only nit is config.json doesn't need to have a . prefix - no reason to hide it.