Add ports into pipeline backend step model (#2656)

Closes #2655.


[Pipeline](https://woodpecker-ci.org/docs/next/usage/services#complete-pipeline-example):
```yaml
services:
  database:
    image: mysql
    environment:
      - MYSQL_DATABASE=test
      - MYSQL_ROOT_PASSWORD=example
    ports:
      - 3306

steps:
  get-version:
    image: ubuntu
    commands:
      - ( apt update && apt dist-upgrade -y && apt install -y mysql-client 2>&1 )> /dev/null
      - sleep 60s # need to wait for mysql-server init
      - echo 'SHOW VARIABLES LIKE "version"' | mysql -uroot -hdatabase test -pexample
```

Service:
```yaml
apiVersion: v1
kind: Service
metadata:
  name: wp-01hdq6gbkw1mn6k1655fs3rntf-0-services-0
  namespace: woodpecker-runtime
  ...
  selfLink: >-
    /api/v1/namespaces/woodpecker-runtime/services/wp-01hdq6gbkw1mn6k1655fs3rntf-0-services-0
status:
  loadBalancer: {}
spec:
  ports:
    - protocol: TCP
      port: 3306
      targetPort: 3306
  selector:
    step: database
  clusterIP: 10.43.180.120
  clusterIPs:
    - 10.43.180.120
  type: ClusterIP
  sessionAffinity: None
  ipFamilies:
    - IPv4
  ipFamilyPolicy: SingleStack
  internalTrafficPolicy: Cluster
```

pipeline/backend/kubernetes/service_test.go

@@ -54,7 +54,7 @@ func TestService(t *testing.T) {
 	  }
 	}`
 
-	s, _ := Service("foo", "bar", "baz", []string{"1", "2", "3"})
+	s, _ := Service("foo", "bar", "baz", []uint16{1, 2, 3})
 	j, err := json.Marshal(s)
 	assert.NoError(t, err)
 	assert.JSONEq(t, expected, string(j))