Allow clients to fade-out by streaming a short amount of time after muting.

[luar123]

Is your feature request related to a problem? Please describe. Some clients have a fade-out/soft mute feature where the volume is decreased when the client is muted (e.g. esp32-snapclient https://github.com/jorgenkraghjakobsen/snapclient or https://github.com/CarlosDerSeher/snapclient). However, to make this work send_to_muted needs to be true and therefore a lot of unnecessary network traffic is generated.

Describe the solution you'd like send_to_muted should take an integer value as input that defines for how many seconds (or ms or chunks) the server should send data to muted clients. -1 or 99 could be used as the old send_to_muted = true. And maybe a new setting should be used instead of re-using send_to_muted?

Additional context I have already done a quick implementation in StreamServer::onChunkEncoded re-using send_to_muted and I could create a PR. Please let me know what you think.