Add host-aware FluentFTP config callback to factories

[Copilot]

FluentFTP client creation could not be customized per target FTP host. This change adds a host-aware configuration callback so callers can mutate FluentFTP.FtpConfig during client construction in both pooled and non-pooled flows.

• What changed

  • Added overloads to FluentFtpClientFactory and FluentFtpClientPoolFactory accepting:

    Action<string, FluentFTP.FtpConfig>

  • Propagated the callback through FluentFtpClientConnectionFactory into FluentFtpClientConnection.
  • Applied the callback during AsyncFtpClient creation, with the FTP host passed alongside the mutable FluentFTP.FtpConfig.

• Behavior

  • Configuration can now vary by host without changing shared defaults.
  • The same extension point is available for both direct client creation and pooled connections.

• API shape

  var factory = new FluentFtpClientFactory((host, config) =>
  {
      if (host == "partner-a.example.com")
      {
          config.SocketKeepAlive = false;
      }
  });

• Scope

  • Preserves existing constructors and default behavior when no callback is provided.
  • Extends only the FluentFTP-backed implementation; no changes to the core FTP abstractions were required.