Apps: Support Project Assignment

[danaelhe]

Adds support for specifying a project ID for a app. Addresses #1112

Tried it out locally and was able to confirm an App was created in specified project:

terraform {
  required_providers {
    digitalocean = {
      source = "digitalocean/digitalocean"
      version = ">= 2.8.0"
    }
  }
}

provider "digitalocean" {
  # You need to set this in your .bashrc
  # export DIGITALOCEAN_TOKEN="Your API TOKEN"
  #
}

resource "digitalocean_project" "foobar" {
  name = "test_apps"
}

resource "digitalocean_app" "golang-sample" {
  project_id = digitalocean_project.foobar.id
  spec {
    name   = "golang-sample"
    region = "ams"

    service {
      name               = "go-service"
      environment_slug   = "go"
      instance_count     = 1
      instance_size_slug = "professional-xs"

      git {
        repo_clone_url = "https://github.com/digitalocean/sample-golang.git"
        branch         = "main"
      }
    }
  }
}

[andrewsomething]

Looks great. One small nitpick in the data source schema.