flvmux: Allow requesting streamable pads after header is written

flvmux: Allow requesting streamable pads after header is written

Allows us to request pads after writing header for streamable flv's.

For non-streamable it doesn't make sense to request a new pad after
writing the header, because the headers have been written already and we
can't add the new stream. But for streamable, any clients that connect
after the new pad has been added will be able to see both streams.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/572>