feat(onesignal): add in-app messages methods

[cristianofromagio]

Fix issue #3478

On the documentation the OSInAppMessageAction Class is listed as being camelCase but the result properties of the Cordova SDK method that I tested returns snake_case (likely inspired by a json conversion as also seem on the Flutter SDK), so I opted to set the interface with the snake_case notation.

Example of a tested OSInAppMessageAction result object:

[ng] [console.log]: "in-app message clicked"
[ng] [console.log]: {
[ng]   "click_url": "",
[ng]   "first_click": true,
[ng]   "closes_message": true,
[ng]   "outcomes": []
[ng] }