encodebin: Add APIs to set element properties on encoding profiles

User often want to set encoder properties on encoding profiles, this introduces a way to easily 'preset' properties when defining the profile. This uses GstStructure to define those properties the same way it is done in splitmux for example as it makes simple to handle.

This also defines a more complex structure type where we can map a set of properties to set depending on the muxer/encoder factory that has been picked by EncodeBin so it is quite flexible.