chore(issue 75): integration tests for attributes and consumption of namespaces table within attributes protos/schema

[jakedoublev]

Closes: #75
Now the following get/list requests for attributes look like this:

Request: 
grpcurl -plaintext localhost:9000 attributes.AttributesService/ListAttributes

Response: 
{
  "attributes": [
    {
      "id": "ebec024c-9ec2-4465-9b5a-dc643abf07cb",
      "metadata": {
        "createdAt": "2024-01-26T21:45:15.978314Z",
        "updatedAt": "2024-01-26T21:45:15.978314Z"
      },
      "namespace": {
        "id": "348f3698-a0c9-4ed6-a90c-e589aaf59c57",
        "name": "example.com"
      },
      "name": "attribute_name",
      "rule": "ATTRIBUTE_RULE_TYPE_ENUM_ALL_OF"
    }
  ]
}