amazon-import -> Import as snapshot?

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#### Description

Instead of only having the possibility of importing the artifacts as an amazon AMI, which has a lot of requirements, we should be able to import the artifact as a snapshot, which has almost no requirements and its not even postprocessed by aws.

It could even have an extra step to register the AMI from that snapshot afterwards thus turning the snapshot into an AMI

#### Use Case(s)

Any OS that does not adhere to the requirements list from importing images[0] could benefit from this. In our case this arise from building https://github.com/rancher-sandbox/cOS-toolkit and trying to use packer while building our artifacts for the different cloud providers. We tried to use amazon-import to upload the resulting OVA images but it kept failing until we used the import-snapshot method.

[0] https://docs.aws.amazon.com/vm-import/latest/userguide/vmie_prereqs.html

#### Potential configuration

```diff
{
"type": "amazon-import",
"access_key": "YOUR KEY HERE",
"secret_key": "YOUR SECRET KEY HERE",
"region": "us-east-1",
"s3_bucket_name": "importbucket",
"license_type": "BYOL",
+"snapshot": true,
+"register_snapshot": true,
"tags": {
"Description": "packer amazon-import {{timestamp}}"
}
}

```

#### Potential References
https://docs.aws.amazon.com/vm-import/latest/userguide/vmimport-import-snapshot.html
https://docs.aws.amazon.com/cli/latest/reference/ec2/register-image.html