Add functionality to export retrospective as PDF

[Mewbi]

This pull request adds support for exporting retrospectives as PDF files in addition to the existing JSON and Markdown formats. The most important changes are grouped below:

PDF Export Functionality:

• Added a new method ConvertRetrospectiveToPDF in internal/service/convert.go that generates a PDF representation of a retrospective using the github.com/go-pdf/fpdf library. The PDF includes the title, description, creation date, questions, and answers, formatted for readability.
• Updated the export handler in internal/server/server.go to handle the new PDF export type: it calls the new conversion method, sets the correct headers, and returns the PDF file as a download.
• The PDF generate should be similar as the following:

API and Type Updates:

• Added ExportTypePDF to the ExportType enum in types/retrospective.go to support the new export option.
• Updated the export request validation in types/validations.go to accept PDF as a valid export type.
• Enhanced the API documentation and response description in internal/server/server.go to reflect that the export endpoint can now return a PDF file.

Dependency Management:

• Added github.com/go-pdf/fpdf as an indirect dependency in go.mod to enable PDF generation.