Implements private nat-gateway

[haarchri]

Signed-off-by: haarchri [email protected]

Description of your changes

added option to create private-natgateway - to use the natgateway for transit-gateway without internet-gateway

https://aws.amazon.com/de/about-aws/whats-new/2021/06/aws-removes-nat-gateways-dependence-on-internet-gateway-for-private-communications/

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

kubectl apply -f examples/ec2/private-natgateway.yaml  
kubectl apply -f examples/ec2/natgateway.yaml 

kubectl delete -f examples/ec2/private-natgateway.yaml  
kubectl delete -f examples/ec2/natgateway.yaml 

 kubectl describe natgateway.ec2.aws.crossplane.io/private-natgateway
Name:         private-natgateway
Namespace:    
Labels:       <none>
Annotations:  crossplane.io/external-create-pending: 2021-10-20T22:12:00+02:00
              crossplane.io/external-create-succeeded: 2021-10-20T22:12:01+02:00
              crossplane.io/external-name: nat-02e64185616ea2e65
API Version:  ec2.aws.crossplane.io/v1beta1
Kind:         NATGateway
Metadata:
  Creation Timestamp:  2021-10-20T20:12:00Z
  Finalizers:
    finalizer.managedresource.crossplane.io
  Generation:        2
  Resource Version:  6916716
  Self Link:         /apis/ec2.aws.crossplane.io/v1beta1/natgateways/private-natgateway
  UID:               471d3148-34bf-4e24-947a-aa93bf55442d
Spec:
  Deletion Policy:  Delete
  For Provider:
    Connectivity Type: private
    Region:     eu-central-1
    Subnet Id:  subnet-f2ccb898
    Subnet Id Ref:
      Name:  sample-subnet1
    Tags:
      Key:    Name
      Value:  private-natgateway
  Provider Config Ref:
    Name:  default
Status:
  At Provider:
    Create Time:  2021-10-20T20:12:01Z
    Nat Gateway Addresses:
      Network Interface Id:  eni-0acfb3cef703bda43
      Private Ip:            172.31.17.51
    Nat Gateway Id:          nat-02e64185616ea2e65
    State:                   available
    Vpc Id:                  vpc-aab73bc0
  Conditions:
    Last Transition Time:  2021-10-20T20:14:02Z
    Reason:                Available
    Status:                True
    Type:                  Ready
    Last Transition Time:  2021-10-20T20:12:01Z
    Reason:                ReconcileSuccess
    Status:                True
    Type:                  Synced

NAME                                                  READY   SYNCED   ID                      VPC            SUBNET            ALLOCATION ID       AGE
natgateway.ec2.aws.crossplane.io/private-natgateway   True    True     nat-02e64185616ea2e65   vpc-aab73bc0   subnet-f2ccb898                       37m
natgateway.ec2.aws.crossplane.io/sample-natgateway    True    True     nat-0202f960550150def   vpc-aab73bc0   subnet-f2ccb898   eipalloc-39dc1c38   36m