Add `initial_volume` option

[DarkMatterMatt]

This PR adds support for changing the default volume. It's completely opt-in and does not change any existing behaviour

Currently the default is at 90% volume. While I appreciate my speakers' ability to be loud, I wanted to be able to default to a more sensible volume.

See the librespot documentation for more information on how initial-volume works

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To test this option:

1. Change the value in the config, e.g. 50 (50%)
2. Save changes & restart the addon a. If the log level is debug, see that initial-volume is present in the Command line argument(s) section
3. Connect to Home Assistant from another device, see that the volume is 50%