Add endpoint to export retrospective as JSON or Markdown

[Mewbi]

This pull request introduces an export feature for retrospectives, allowing users to export retrospective data as either JSON or Markdown. The main changes include a new API endpoint, request/response types, validation, and the Markdown conversion logic. Below are the most important changes grouped by theme:

API Enhancements:

• Added a new endpoint /retrospective/export to export retrospectives in either JSON or Markdown format, including request validation and appropriate error handling. [1] [2]
• Introduced the RetrospectiveExportRequest type and ExportType enum to specify export parameters and supported formats. [1] [2]

Business Logic:

• Implemented the ConvertRetrospectiveToMarkdown method in the service package to generate a Markdown-formatted export of a retrospective.

Validation:

• Added validation logic for RetrospectiveExportRequest, ensuring valid retrospective IDs and export types.

Dependency Injection:

• Updated the ScheduleParams struct to include a Logger dependency, ensuring logging is available throughout the schedule lifecycle.