Modal Components v2 Single-select Typeconverters, Rest, and Patches

[Cenngo]

This PR adds:

• Newly implemented modal components support to interaction framework rest modules which were missing
• Adds typeconverters for single IUser/IRole/IChannel/IMentionable/IAttachment entities to be used in tandem with single-select and single-file file upload componentes
• Misc fixes (eg. NRE on RespondWithModalAsync())

Breaking Changes:

• RespondWithModal() sync extension methods for RestInteractions are renamed and made async. However, this shouldn't affect regular day-to-day usage, as it was mainly used within the RestInteractionModuleBase.RespondWithModalAsync() method, whose signature remains unchanged.