It should be possible to specify the channels for which a value handler can set values to

[LucaGallinari]

This could be seen as an extension of #112.
It should be possible to specify one or more channels as an option of value handlers, so that these value handlers can set values only on channels specified in its options and not on all. This is necessary due to a limitation of Sylius: it's not possible to specify different values by channel for the same product attribute.
For example, if we have two channels on Akeneo, one for "Europe" and on for the "Rest of the world", we can set different values for the same attribute on Akeneo. But on Sylius this cannot be done, and the only workaround is to duplicate the attribute and use one for the "Europe" channel and one for the "Rest of the world". But we need to be able to import values from Akeneo accordingly, so that we set values for each attribute only for its "related" channel(s).