@spec issue_link_token(ExLine.Client.t(), String.t()) ::
  {:ok, map()} | {:error, ExLine.Error.t()}

Issues a one-time link token for user_id, used to start the account-linking flow.

Returns {:ok, %{"linkToken" => token}} or {:error, ExLine.Error.t()}. The token is valid for ~10 minutes and one-time use.

iex> client = ExLine.Client.new(access_token: "tok", adapter: MyAdapter)
iex> ExLine.Api.AccountLink.issue_link_token(client, "U123")
{:ok, %{"linkToken" => "..."}}

Ref: https://developers.line.biz/en/reference/messaging-api/#issue-link-token