Extend test harness to support envconfig

[THardy98]

What was changed

Add opt-in envconfig support to the shared TypeScript integration-test harness.

When TEMPORAL_TEST_ENV_CONFIG_SERVER is truthy, the harness loads the server address, namespace, API key, TLS configuration, and gRPC metadata through @temporalio/envconfig. These settings are propagated to client, worker, and additional native connections.

When the opt-in is absent, existing behavior is preserved: TEMPORAL_SERVICE_ADDRESS selects the legacy existing-server path, otherwise the harness starts a local server.

This PR intentionally does not migrate any existing integration tests to envconfig. It adds dedicated integration coverage for the new capability only.

Why?

This enables integration tests to target arbitrary Temporal server environments, including local, staging, release, and Temporal Cloud environments.

This is the first SDK implementation of the approach intended to be replicated across Temporal SDKs.

Part of temporalio/features#851

How was this tested?

• An integration test runs a Workflow against an envconfig-selected server using the standard shared test harness and a non-default namespace.
• Focused tests cover connection-option propagation, opt-in precedence, legacy fallback, and TLS environment-variable semantics.
• Existing integration tests continue using their current configuration.

Documentation

No user-facing documentation changes are required. This PR changes internal integration-test infrastructure.