Behaviour for PostgreSQL connection pooling and query execution.

Enables BroadwayPgQue to operate completely independently of Ecto, supporting:

• BroadwayPgQue.Pool.Postgrex - Pure Postgrex connection or dedicated pool (zero Ecto dependency).
• BroadwayPgQue.Pool.Ecto - Uses an existing Ecto.Repo pool.
• Custom pooling modules (e.g., custom DBConnection, PgBouncer pooler, or test mocks).

Example Custom Pool

defmodule MyCustomPool do
  @behaviour BroadwayPgQue.Pool

  @impl true
  def init(opts) do
    {:ok, opts[:conn_name]}
  end

  @impl true
  def query(conn_name, sql, params, opts) do
    Postgrex.query(conn_name, sql, params, opts)
  end
end