App: internal ports array attribute

[rienafairefr]

The digitalocean_app spec was also missing the internal_ports attribute on a service, this should fix this. Acceptance test (TestAccDigitalOceanApp_InternalPort) is passing
internal_ports are specified like internal_ports = ["5000"], maybe an array syntax

internal_port {
   port = 5000
}
internal_port {
   port = 2000
}

would be preferred, similarly to the routes attribute. not sure.

[andrewsomething]

passing internal_ports are specified like internal_ports = ["5000"], maybe an array syntax

I think you've got it right as it currently is. It matches how the yaml spec spec treats them:

  internal_ports:
  - 5000
  - 3000
  routes:
  - path: /
  - path: /foo

https://www.digitalocean.com/docs/app-platform/references/app-specification-reference/

routes could grow additional attributes while internal_ports will always be a list of integers.

[andrewsomething]

Looks great. I left one comment in line. Think we can get rid of appSpecInternalPortSchema()

[andrewsomething]

Thanks again for the great contributions!