Add Export/Import Data functionality for column classifications

[Copilot]

This PR implements comprehensive export and import functionality for SQL Server column classification data, allowing users to backup, restore, and migrate their data classification configurations.

Features Added

Export Data

• New Action: ExportData available on Database entities
• Exports all column classification data to structured JSON format
• Includes information types, sensitivity labels, and descriptions
• Creates timestamped files: ClassifyData_Export_{DatabaseName}_{Timestamp}.json

Import Data

• New Action: ImportData available on Database entities
• Imports classification data from JSON format matching export structure
• Validates data integrity and column existence before applying changes
• Provides detailed success/error reporting with counts and specific error messages
• Only applies non-empty classification values to preserve existing data

Test Export

• New Action: TestExportData for validation and testing
• Verifies database connectivity and column discovery without creating files
• Useful for troubleshooting and confirming functionality

Technical Implementation

• Added ClassificationData and ColumnClassification DTOs for structured data transfer
• Enhanced DatabaseActions class with three new action methods
• Updated model.json to register actions in the Vidyano UI framework
• Comprehensive error handling and validation throughout
• SQL injection protection using parameterized queries
• Maintains transaction consistency and data integrity

JSON Export Format

{
  "ExportedAt": "2024-01-15T14:30:00Z",
  "DatabaseName": "SampleDatabase",
  "Columns": [
    {
      "Schema": "dbo",
      "Table": "Users", 
      "Column": "Email",
      "Type": "varchar",
      "InformationTypeId": "5C503E21-22C6-81FA-620B-F369B8EC38D1",
      "InformationTypeName": "Contact Info",
      "SensitivityLabelId": "684a0db2-d514-49d8-8c0c-df84a7b083eb",
      "SensitivityLabelName": "General",
      "Description": "User email address"
    }
  ]
}

Documentation

• Added comprehensive EXPORT_IMPORT.md with usage instructions and technical details
• Included sample JSON export file demonstrating the data format
• Updated main README.md to highlight the new functionality
• Provided JSON schema documentation and usage examples

Use Cases

• Backup/Restore: Export classifications before major schema changes
• Environment Migration: Move classifications from dev to production
• Bulk Operations: Apply consistent classifications across multiple databases
• Compliance Reporting: Generate structured reports of data classification status
• Integration: Provide data for external classification management systems

Safety Features

• Validates column existence before applying classifications
• Skips columns with empty/null classification values
• Provides detailed error reporting for failed operations
• Maintains existing data integrity during import operations
• Uses parameterized SQL queries to prevent injection attacks

The implementation follows existing code patterns and integrates seamlessly with the current Vidyano-based application architecture while providing robust new capabilities for classification data management.

Fixes #8.

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