Add Mailchimp Integration for Elsa Workflows

[Copilot]

This PR implements a comprehensive Mailchimp integration module for Elsa Workflows, enabling workflows to interact with Mailchimp for campaign management, list handling, member management, and event monitoring.

🚀 Features

Core Activities

• List Management: Create, read, update, and delete lists
• Member Management: Add/update subscribers, retrieve member details, and manage subscriptions
• Campaign Management: Full CRUD operations for email campaigns
• Event Monitoring: Webhook triggers for list events, subscriber changes, and campaign activities
• Generic API Access: MakeAPICall activity for arbitrary Mailchimp API operations

Architecture

• Thread-safe client factory with singleton pattern for efficient API client management
• Base activity classes (MailchimpActivity, MailchimpTriggerActivity) following established patterns
• Proper dependency injection setup with MailchimpFeature
• Easy integration via UseMailchimp() extension method

📋 Implementation Details

Activities Implemented

Category	Activities
Lists	CreateList, GetList, UpdateList, DeleteList
Members	AddUpdateListMember, GetSubscriber, DeleteSubscriber
Campaigns	CreateCampaign, GetCampaign, UpdateCampaign, DeleteCampaign
Events	WatchLists, WatchSubscribers, WatchCampaigns
Core	MakeAPICall

Usage Example

// Register the integration
builder.Services.AddElsa(elsa => elsa.UseMailchimp());

// Use in a workflow
public class AddSubscriberWorkflow : IWorkflow
{
    public void Build(IWorkflowBuilder builder)
    {
        builder
            .StartWith<AddUpdateListMember>(activity =>
            {
                activity.ApiKey = "your-mailchimp-api-key";
                activity.ListId = "your-list-id";
                activity.EmailAddress = "[email protected]";
                activity.Status = "subscribed";
            });
    }
}

🧪 Testing

• Comprehensive unit tests for core services and features
• Test coverage for client factory thread safety and dependency injection

📚 Documentation

• Complete README.md with usage examples and API references
• Inline documentation for all activities with input/output descriptions
• Integration instructions and authentication guidance

🔧 Technical Notes

• Uses MailChimp.Net library for API interactions
• Supports authentication via Mailchimp API keys
• Follows established patterns from existing integrations (GitHub, Slack, Email)
• All webhook triggers include placeholder implementations pending webhook infrastructure

Fixes #10.

Warning

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I tried to connect to the following addresses, but was blocked by firewall rules:

• f.feedz.io
  • Triggering command: dotnet build (dns block)
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• http://168.63.129.16:80/machine/
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