Add support for app platform type on digitalocean_database_firewall resources

[saccy]

Is your feature request related to a problem? Please describe.

DigitalOcean now supports app platform as a trusted source for database clusters but this option is not available in terraform

Describe the solution you'd like

An option to use app_platform as a recognised type when defining a trusted source for a DO database cluster in terraform. If this is already available then doco needs to be updated.

Additional context

https://registry.terraform.io/providers/digitalocean/digitalocean/latest/docs/resources/database_firewall