Add support to set playback rate

Hey, Logseq is becoming more and more my defacto podcast player and I love it. There is just one thing that trips me up.

I’m used to listen to podcast in 1.5 playback speed. Although the audio player allows me to adjust the playback speed manually, it does not persist the value. I would love to be able to set the playback speed as an argument like this:

![](random-audio.mp3){playbackRate=1.5}

The API / markdown flavour is up for discussion. I remember seeing a similar example with curly braces to specify the dimensions of an image in one of the many markdown flavours.