Feature Request: Implement endpoint Workflow Stage Changes

[roberto-butti]

Implement a specific endpoint for Workflow Stage Changes.
Even if it is possible to call the API endpoint via ManagementApi Endpoint class, implement specific and more strict classes:

• WorkflowStageChangesApi.php namespace Storyblok\ManagementApi\Endpoints;
• WorkflowStageChangeData.php namespace Storyblok\ManagementApi\Data;
• WorkflowStagesChangeData.php namespace Storyblok\ManagementApi\Data;

Management API reference: https://www.storyblok.com/docs/api/management/workflow-stage-changes/

[roberto-butti]

At the moment, it’s already feasible to interact with workflow stage changes via the generic ManagementApi class, e.g.:

$response = new ManagementApi($client)->post(
    "spaces/{$spaceId}/workflow_stage_changes",
    [
        "workflow_stage_change" => [
            "workflow_stage_id" => $workflowStageId,
            "story_id" => $story->get("id"),
            "due_date" => null
        ]
    ]
);

$wsc = $response->data();

While this works, the approach requires manually constructing paths and payloads, and it lacks type safety or autocompletion support.

By contrast, a dedicated endpoint class like WorkflowStageChangesApi could provide a cleaner, stricter interface:

$workflowStageChangeApi = new WorkflowStageChangeApi($client);
$wsc = $workflowStageChangeApi->create(new WorkflowStageChangeData(
    workflowStageId: $workflowStageId,
    storyId: $story->get("id"),
    dueDate: null
));

This would:

• Reduce boilerplate when creating or managing workflow stage changes.
• Make the API self-explanatory through method names and data classes.
• Improve developer experience (DX) with stricter typing and IDE support.