[Feature Request] Add option to set location in media

[Timmiej93]

I'd like to be able to pick a point in a song / podcast that I want the media player to start at, instead of starting at 0:00. This would be a massive help for using this for playing sleep/wake up sounds, where you (usually) want to start at a random point in the multi-hour podcast. My suggestion would be something like this:

service: spotcast.start
data:
  entity_id: media_player.player
  uri: spotify:episode:4bsCcf9GAUILks2f4Ph0YY
  start_offset: 7200 # Start at the 2:00:00 mark

[siemon-geeroms]

my PR #438 will allow this