With the content definitions api you can “remotely manage definitions, such as Content types, Property Groups, and Property Data Types without deploying code to the Optimizely application or using the admin user interface.”
Personally, as a dinosaur, I think I will still do it the “old” way by creating content types in code, but in case I would need to create json “by hand” I made some Resharper Templates and Visual Studio Code Snippets for content types, property data types, etc.
I have put them on GitHub in case you might want/need to use them.
EPiServer Development 