Add missing support for App Platform CRUD operations

[bentranter]

Add support for five App Platform CRUD operations defined here:

• Create https://docs.digitalocean.com/reference/api/api-reference/#operation/apps_create
• Get https://docs.digitalocean.com/reference/api/api-reference/#operation/apps_get
• List https://docs.digitalocean.com/reference/api/api-reference/#operation/apps_list
• Update https://docs.digitalocean.com/reference/api/api-reference/#operation/apps_update
• Delete https://docs.digitalocean.com/reference/api/api-reference/#operation/apps_delete

There is currently no support for App Platform, so you'll need to create new files in lib/droplet_kit/mappings, lib/droplet_kit/models, and lib/droplet_kit/resources. You'll also need to add the new resource in lib/droplet_kit/client.rb, add the appropriate autoloads in lib/droplet_kit.rb, and test converage in spec/lib. Finally, you'll need to add documentation for these new methods in the README.md file.

A good example of a recently added new resource can be seen in #318.