Support association of RouteTable to Subnet in ec2-controller

[RedbackThomson]

Is your feature request related to a problem?
The EC2 controller currently supports creating a Subnet and a RouteTable resource, but does not support a way to associate the two. Since a SubnetRouteTableAssociation does not have a unique identifier, it does not warrant its own resource. It would be more appropriate to place a list of associated RouteTable as a field in the spec of Subnet.

Describe the solution you'd like

apiVersion: ec2.services.k8s.aws/v1alpha1
kind: Subnet
metadata:
  name: subnet-1
spec:
  routeTableIDs:
  - rt-12345
  - rt-67890

with a corresponding resource reference:

apiVersion: ec2.services.k8s.aws/v1alpha1
kind: Subnet
metadata:
  name: subnet-2
spec:
  routeTableRefs:
  - from:
      name: my-route-table-1
  - from:
      name: my-route-table-2