@spec request(Sendio.Client.t(), Keyword.t()) ::
  {:ok, %{body: map(), status: pos_integer()}} | {:error, any()}

Sends a request to a send.io API endpoint, given list of request opts.

opts holds :method, :url, :body, :query, opts: [path_params: ...] and :headers, a list of extra {name, value} request headers (for example {"idempotency-key", key}).