POST /v2/clients

Create a new client.

Request:
{
  "data": {
    "type": "client",
    "attributes": {
      "name": "string (required)"
    },
    "relationships": {
      "parent": {
        "data": { "type": "client", "id": "string" }
      }
    }
  }
}

Set `parent.data` to nest the new client under an existing one. Omit
the `parent` relationship entirely (or send `"data": null`) to create
a top-level client.

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "client",
    "attributes": {
      "name": "string"
    },
    "relationships": {
      "parent":             { "data": { "type": "client", "id": "string" } | null },
      "children":           { "data": [] },
      "legalEntities":      { "data": [] },
      "reportingEntities":  { "data": [] }
    }
  }
}