feat: Added follow_up, co_follow_up functions and documentation improvements

[Feinq]

This PR introduces follow_up and co_follow_up functions on dpp::interaction_create_t.

This change also includes two improvements found during implementation:

• Documentation: Corrected several doxygen comments in dpp/cluster.h that incorrectly referred to "slash commands" instead of the more general "interactions" for interaction-related functions, since follow ups naturally work for other interactions such as button interactions as well.
• Performance: The for-loop in edit_original_response now uses const& to avoid expensive copies of file data when editing a message with attachments.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

[github-actions]

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