add everyone_only as a filter for EBS snapshots

[ikraemer-dd]

This PR is meant to solve the following issue: #4072

I also tested this against an AWS account where I had created 3 snapshots in ou-test: 1 public, 1 shared with another account and 1 private. I use this policy:

policies:
    - name: aws_ebs-shared-public-snaphots
      description: Find public snapshots
      resource: aws.ebs-snapshot
      filters:
        - type: cross-account
          everyone_only: True

And I get:

(.venv) $ cat resources.json
[
  {
    "Description": "public-snapshot",
    "Encrypted": false,
    "OwnerId": "REDACTED",
    "Progress": "100%",
    "SnapshotId": "snap-REDACTED",
    "StartTime": "2023-05-04T16:38:33.861000+00:00",
    "State": "completed",
    "VolumeId": "vol-REDACTED",
    "VolumeSize": 8,
    "StorageTier": "standard",
    "c7n:CrossAccountViolations": [
      "all"
    ]
  }
]

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ikraemer-dd / name: Isabelle Kraemer (3f81ebb)

[kapilt]

Thanks for the pr, lgtm, one minor to address.