Feature Request: Dynamic Workflow Variable Initialization

**Summary**
Enable dynamic initialization of workflow variables from API requests to support parameterized workflow execution.

**Problem Statement**
Currently, workflows can only initialize variables with static values using SetVariable actions in the OnConversationStart trigger. For example:

```yaml
- kind: SetVariable
id: set_customer_id
variable: Local.CustomerId
value: "cust-1" # Hard-coded value
```

This creates challenges for:

- Testing: Cannot parameterize test scenarios with different customer/account IDs
- Reusability: Same workflow cannot be used with different input parameters
- Evaluation: Cannot run automated test suites with varied inputs

**Current Workarounds Attempted**
Metadata in ChatMessage: Tried passing variables in [message.metadata] but they're not accessible in workflow expressions
System.Variables: Attempted =System.Variables.CustomerId, but no mechanism exists to populate this from API requests
extra_body parameters: Explored various [extra_body] configurations but no documented approach works

Option 1: Structured Inputs (Preferred)
Allow passing workflow variables via structured_inputs in the API request:

```python
response = await openai_client.responses.create(
conversation=conversation_id,
input="User message",
extra_body={
"agent": {"name": "workflow-name", "type": "agent_reference"},
"structured_inputs": {
"CustomerId": "cust-3",
"AccountId": "acct-125"
}
}
)
```

Workflow could access these as:

```yaml
- kind: SetVariable
variable: Local.CustomerId
value: =System.StructuredInputs.CustomerId
```

Or automatically make them available without SetVariable:

```yaml
- kind: InvokeAzureAgent
input:
arguments:
CustomerId: =Input.CustomerId
```

**Priority**
High - This is a fundamental requirement for production workflows and automated testing scenarios.