Changing notification settings through the WordPress REST API

[nicucalcea]

Description:

I'm publishing content through the WordPress REST API. I would like to toggle the "Send notification on post publish" checkmark on a per-post basis, but I can't see a way to do that through the API.

Steps to Reproduce Issue:

1. Upload content through the API (POST to example.com/wp-json/wp/v2/posts). I'm personally doing it in R.
2. I'm setting the title, description, post status, featured image, etc. in the body of the POST.
3. However, I don't know how to customise any of the onesignal settings in the body of the POST or in any other programmable way.

[rgomezp]

Duplicate of #267