@spec whoami(Dnsimple.Client.t(), Keyword.t()) ::
  {:ok | :error, Dnsimple.Response.t()}

Returns information about the currently authenticated user and/or account.

See:

• https://developer.dnsimple.com/v2/identity/#whoami

Examples:

client = %Dnsimple.Client{access_token: "a1b2c3d4"}
{:ok, response} = Dnsimple.Identity.whoami(client)