feat(eml): add local EmlConverter supporting header extraction, body parsing, and recursive attachment conversion

[saitejabandaru-in]

Description

This PR introduces a local EmlConverter to natively support parsing .eml (email) files.

Key Changes

1. Added EmlConverter: Uses the built-in Python email module to extract standard email headers (From, To, Subject, Date) and the email body (with HTML bodies automatically converted to Markdown).
2. Recursive Attachment Conversion: Iterates through message attachments, recursively parsing them using the main MarkItDown instance, and appends the converted content under ## Attachment: <filename> headings (matching the pattern used by ZipConverter).
3. Unit Tests: Added a robust end-to-end unit test to verify header extraction, body parsing, and recursive attachment handling.

Closes #1662.