audioconvert: do _port_set_param on fmtconvert if convert mode

This MR fixes the audioconvert plugin (and therefore audioadapter) so that it can be configured in convert mode.

This is needed so that client nodes with a different audio format than the one support by the device can be linked with device nodes in passthrough mode. In this case, the client node would be configured in convert mode.

This can be tested using my latest changes in the example media-session, which adds a new "alsa-passthrough" option: https://gitlab.freedesktop.org/julian/pipewire/-/commits/alsa-passthrough