[Core] Implement [p]mydata getdata

Currently, the `[p]mydata getdata` command in `redbot/core/core_commands.py` is a stub which notes that the command will be available in the future. This issue serves as a place to track the progress towards the implementation of that command, as well as a place for discussion on how to do so.

The main consideration in implementing this command is the process of compiling and presenting the data to the user. User data can take many forms, including binary formats such as images. It must be possible for cogs to include all types of data, as well as multiple at the same time. This data must be presented to the user in an easy to access format.