Add a new Orchestrator "promptflow" that calls the prompt flow

[tectonia]

Required by #406

Description

Create a prompt flow orchestrator.

Tasks

• [x] Process messages
• [x] Implement keys based authentication
• [x] Test with infrastructure written in #1003
  • [x] Update local .env
  • [x] Update orchestrator to match infra
  • [x] Set environment variables for app services
  • [x] Add role assignments for web app to workspace, endpoint and deployment
• [x] Write tests

[tectonia]

The MLClient online_endpoints.invoke method detects the Default Azure Credential whether that is the key or RBAC token. However, this has not been tested for RBAC and further changes might have to be made on provisioning side. This means if you choose to select prompt_flow as ORCHESTRATION_STRATEGY, you might run into issues if you decide to set AZURE_AUTHTYPE as rbac. From docs: "If you plan to use the SDK to invoke the endpoint, you're not required to get the keys or token for data plane operations explicitly, as the SDK handles it for you."_