fdkaacdec: Add Converter class to hint gst-validate

fdkaacdec have minimal conversion capability, adding the Converter class allow gst-validate to behave properly and not spit an error when it notice that the number of channels or rate miss-match in and out.

Same logic as with opusdec, see: https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1142>