[Refactor] Standardize plugin architecture for workflows

[fderuiter]

Context

The SDK currently loads the imednet-workflows plugin via a dynamic import_module inside a try/except block, returning Any. This defeats static typing, breaks IDE autocomplete for sdk.workflows, and introduces a hidden dependency anti-pattern.

Implementation Plan

• Refactor the workflow loading mechanism to use importlib.metadata.entry_points.
• The imednet-workflows package should register itself to an imednet.plugins entry point in its pyproject.toml.
• Ensure Mypy/Pyright can correctly infer the types of loaded workflows.

Acceptance Criteria

• import_module("imednet_workflows.namespace") is replaced with standard entry point discovery.
• Plugin registration is handled via pyproject.toml in the workflows package.
• Static typing is preserved for the .workflows namespace.