Fortunately, there is a quick and hacky solution!
Imagine that we have a list of servers and their associated public keys. To store that data as JSON, we could store it as an array of comma-separated string objects, which might look something like this:
You can verify that this valid JSON with JSONLint.
Then, to retrieve our text block, we just have to join them back together on the newline characters, which we can do like this:
And that's it! Now you can store multiline strings in JSON. It's not the prettiest thing, but it is the simplest given the constraints of JSON.