This PR allows users to assume different AWS roles when interacting with
AWS EKS API. It allows users to proxy EKS clusters in different AWS
accounts using the same Teleport Kubernetes Service.

Example configuration:

```yaml

kubernetes_service:
  enabled: true
  resources:
  - labels:
      'a': 'b'
    aws:
      assume_role_arn: "arn:aws:iam::0987654321:role/KubeAccess"
      external_id: "0987654321"
  - labels:
      'c': 'd'
    aws:
      assume_role_arn: "arn:aws:iam::123456789012:role/KubeAccess"
      external_id: "123456789012"

```