Behaviour for a pluggable Slack Web API HTTP client.

Every function generated under Slack.Web.* calls post!/2 on the module
configured as :web_http_client (default: Slack.Web.DefaultClient).
Implementing this behaviour lets you inject auth headers, add retries,
decorate responses, route requests through a proxy, or instrument calls
for telemetry without touching the generated code.

The return value of post!/2 is handed back to the caller of the
Slack.Web.* function unchanged — there is no required shape, so a custom
client is free to wrap the response in {:ok, _} / {:error, _} tuples
or in a struct of its choosing.

Example

defmodule MyApp.SlackClient do
  @behaviour Slack.Web.Client

  @impl true
  def post!(url, {:form, params}) do
    url
    |> Req.post!(form: params, retry: :transient)
    |> Map.fetch!(:body)
    |> JSON.decode!()
    |> wrap()
  end

  def post!(url, {:multipart, _parts} = body) do
    # Fall back to the default client for multipart uploads.
    Slack.Web.DefaultClient.post!(url, body)
  end

  defp wrap(%{"ok" => true} = body), do: {:ok, body}
  defp wrap(%{"error" => reason} = body), do: {:error, reason, body}
end

# config/runtime.exs
config :slack, :web_http_client, MyApp.SlackClient

See Slack.Web.DefaultClient for the stock implementation built on Req.