AWS OIDC - List EC2: add AWS EC2 Instance ID as label

[marcoandredinis]

Context: #29317

When listing EC2 instances we should be able to tell which ones were already added.
To do so, we should use the instance-id which is unique. We can't use the Spec.CloudMetadata.AWS.InstanceID because predicate can't use fields that are optional.

This PR adds the InstanceID as label.
It uses teleport.dev/instance-id label key.

When creating a Node from WebUI, it will send the same set of labels that was received.
So, WebUI will send this same label.

When listing EC2 instances, WebUI queries the backend for a list of Nodes of type EC2 EICE and the same instance-id and uses the result to show which ones were already added.

Demo

dinis@lenix ~> curl 'https://<cluster>/v1/webapi/sites/lenix/integrations/aws-oidc/teleportdev/ec2' \
                    ....
                     --data-raw '{"region":"eu-west-2"}' | jq

{
  "servers": [
    {
      "kind": "node",
      "tunnel": false,
      "subKind": "openssh-ec2-ice",
      "id": "e822c9bf-bd2a-49e2-a9d3-ff544d000aed",
      "siteId": "lenix",
      "hostname": "ip-172-31-39-120.eu-west-2.compute.internal",
      "addr": "172.31.39.120:22",
      "tags": [
        {
          "name": "Name",
          "value": "MarcoDevTeleport"
        },
        {
          "name": "account-id",
          "value": "278576220453"
        },
        {
          "name": "region",
          "value": "eu-west-2"
        },
        {
          "name": "teleport.dev/instance-id",
          "value": "i-0b1751688b2e4aedf"
        }
      ],
      "sshLogins": [
        "dinis",
        "ubuntu",
        "root"
      ]
    }
  ]
}

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