`Instrument.play()` not public

[stavros11]

Ok, I perfectly agree with this implementation, but I'd say that it is not specific to QM.

I would have liked to have it in the Platform. But this is not really user facing, so we can also change it later on.

Even Instrument.play() should not be public, it is only supposed to be used internally by the Platform; we should consider making only the constructors public (which is not trivial to do it properly, but we can of course define a function passing all the arguments to the constructor, losing the type hints).

Originally posted by @alecandido in https://github.com/qiboteam/qibolab/pull/1008#discussion_r1734168696