Combine callback functions check_item_type(s) and move to specs

[kevinlacaille]

In planet.cli.data and planet.cli.subscriptions, we have identical pairs of callback functions: check_item_type and check_item_types, which were temporarily introduced in #905. These two functions are incredibly similar and we could probably be do with just one of them.

1. Let's explore the possibility of removing one of the two callback functions in order to reduce complexity.
2. Let's move these callback function(s) to planet.specs and update the Data and Subscriptions CLIs, the callback function(s) tests.

[ischneider]

Consider using a click ParamType for reuse