Update Slack build publish plugin artifacts distribution

[thindie]

🚀 Description

Refactor Slack integration to support rich text changelog messages with file uploads. This change removes the deprecated changelog-only functionality and introduces a new unified approach that combines file distribution with rich text changelog display using Slack Block Kit.

📄 Motivation and Context

AT-278 - Slack Build Summary Message
The previous Slack integration had separate functionality for changelog-only messages and file uploads, which created complexity and didn't provide a unified user experience. This change consolidates the functionality to provide rich text messages with file uploads, including user mentions and version information, all in a single Slack message.

🧪 How Has This Been Tested?

• Verified Slack Block Kit JSON generation matches official Slack documentation
• Tested file upload with rich text messages in development environment and real slack channels

📦 Types of changes

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

✅ Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

🔧 Technical Changes

Added

• SlackBlocks.kt - New API for Slack Block Kit rich text formatting
• Support for user mentions in distribution messages
• Version/description display in Slack messages
• Rich text formatting with bold headers and bulleted changelog lists

Removed

• SlackChangelogConfig.kt - Deprecated changelog-only configuration
• SendSlackChangelogTask.kt - Separate changelog task
• SlackWebhookService.kt - Webhook-based changelog sending
• Bot configuration block from BuildPublishSlackExtension

Modified

• SlackUploadService.kt - Enhanced to support rich text blocks
• SlackDistributionConfig.kt - Added user mentions and description fields
• SlackTasksRegistrar.kt - Updated to use distribution-based service lookup
• BuildPublishSlackExtension.kt - Simplified configuration structure

📋 Migration Guide

Before (Deprecated)

buildPublishSlack {
    bot {
        common {
            webhookUrl.set("https://hooks.slack.com/...")
            iconUrl.set("https://i.imgur.com/...")
        }
    }
    changelog {
        // changelog configuration
    }
}

After (New)

buildPublishSlack {
    distributionCommon {
        uploadApiTokenFile.set(File("path/to/slack-token.txt"))
        destinationChannel("builds")
        userMentions("@here", "@channel")
        // distributionDescription("RC v1.3.1990") // Optional: defaults to GitLab tag name
    }
}

🎯 Benefits

• Unified file upload and changelog display in single Slack message
• Rich text formatting with proper Slack Block Kit support
• Simplified configuration structure
• Better user experience with mentions and version information
• Reduced code complexity and maintenance overhead

[thindie]