Add a field indicating a note "type" to the order notes api

[AmandaRiu]

We'd like to create a view in the mobile app that visually distinguishes between different types of notes:

• System status
• Note to customer
• Private note

The API currently only provides customer_note=true|false which is fine for determining a public vs private note, but no data is provided that would allow us to flag a note as "system generated". Wp-admin uses different colors to represent the system generated status so it looks like something we do track:

[aforcier]

It looks like this is already supported in the V3 API, so I assume we'd be able to use it once V3 is rolled out to core.

The notes returned from the API will have an author field, which is either "system" or a username on the site.

[claudiosanches]

Closing since it's already part of v3.