fix(structures): add missing `toJSON` method on Subscription
When writing tests for #11407, it became apparent that I had forgotten to add the `toJSON` method for this structure when I was initially wrote the structure. I have now added this method and it passes when running the tests that I have written for this (which will be merged in a following PR).
Signed-off-by: Asad Humayun <asad.humayun@asadh.io>