Bluetooth: shell: audio: Extend `vcs init` with opt params

Extend the `vcs init` command with optional parameters to set the
initial VCS state including volume level, mute state and step size.
This is needed to pass qualification test cases listed below:
VCS/SR/CP/BV-01-C,
VCS/SR/CP/BV-02-C,
VCS/SR/CP/BV-02-C,
VCS/SR/CP/BV-04-C

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>