Add support for Local Environment Variable Lookup for Variables & Connections in Task SDK

[kaxil]

Currently, the Task Execution API server handles secrets backend logic, which is ideal for scenarios such as:

Deploying tasks in transient or ephemeral environments (e.g., GPU cloud services) where secrets need to be retrieved dynamically via the API.

However, there is a complementary use case where secrets or configurations are already provisioned locally on the worker. For simplicity and flexibility, we should enable the Task SDK to look up Variables and Connections from environment variables local to the worker.

This approach simplifies the logic by:

• Supporting only environment variable lookups on the client-side (avoiding the complexity of implementing a full-fledged Secrets backend in both the client and server).
• Keeping the Task Execution API server as the primary source for dynamic secrets retrieval, but allowing local fallback when needed.

This would delegate the responsibility to the worker provisioner for ensuring that environment variables are populated from external services (e.g., Vault, AWS Secrets Manager, etc.) before task execution for better security.