feat: Workflow Control Plane support

[saurabhjain1592]

Summary

Adds Workflow Control Plane support to the Java SDK, enabling governance gates at workflow step transitions for external orchestrators.

Features

• Workflow control methods: createWorkflow, getWorkflow, stepGate, etc.
• Async variants for all methods (createWorkflowAsync, stepGateAsync, etc.)
• Builder pattern for request types
• Helper methods: isAllowed(), isBlocked(), requiresApproval()

New Files

• types/workflow/WorkflowTypes.java - All workflow types (enums, requests, responses)
• types/workflow/package-info.java - Package documentation

Usage

AxonFlow client = AxonFlow.create(AxonFlowConfig.builder()
    .endpoint("http://localhost:8080")
    .build());

// Create workflow
CreateWorkflowResponse workflow = client.createWorkflow(
    CreateWorkflowRequest.builder()
        .workflowName("my-pipeline")
        .build()
);

// Check gate
StepGateResponse gate = client.stepGate(
    workflow.getWorkflowId(), "step-1",
    StepGateRequest.builder()
        .stepName("Generate")
        .stepType(StepType.LLM_CALL)
        .build()
);

if (gate.isAllowed()) {
    // Execute step
    client.markStepCompleted(workflow.getWorkflowId(), "step-1", null);
}

Test plan

• Unit tests pass
• Test coverage >80%
• Maven build succeeds
• Types compile correctly

Related to getaxonflow/axonflow-enterprise#834