Add command target to interaction response messages

[advaith1]

my last feature drop :)

interaction_metadata can now have target_user or target_message_id for context menu command messages. message command messages will also have message_reference and referenced_messages.

there are also other pre-existing fields that are marked as private, like command_type. maybe those should be marked as public and then documented

I also split up the interaction metadata object into the different structures to reflect how it is stored internally and the API spec. (lib devs will probably like this)