Fixes #14 Dedicated cluster network options and encryption key

[PhilGrayson]

Fixes #14.

• Expose option to define a dedicated cluster's network option (Internet, Private Link, VPC Peering)
• Expose option to define a dedicated cluster's encryption key

This was discovered by submitting a cluster creation request via https://confluent.cloud UI and inspecting the POST body to https://confluent.cloud/api/clusters. This is what the body looks like:

    {
      "config": {
        "name": "cluster_2",
        "account_id": "env-12345",
        "network_ingress": 100,
        "network_egress": 100,
        "storage": 500,
        "durability": "LOW",
        "region": "eu-west-1",
        "service_provider": "aws",
        "organization_id": 12345,
        "deployment": {
          "sku": "DEDICATED",
          "account_id": "env-12345"
        },
        "cku": 1,
        "selected_network_type": "PRIVATE_LINK",
        "encryption_key_id": "arn:aws:kms:eu-west-1:000000000000:key/0000000000000-1111-2222-333333333333"
      }
    }